feat(#28): Gate B [08/10]: AES-GCM settings round-trip (AES-1, AES-2)

- AES-GCM encrypt/decrypt module (app/security/aes.py):
  - get_aes_key: decode base64 AES master key to 32 bytes
  - encrypt_setting: AES-256-GCM with fresh 12-byte nonce per call, returns nonce||ciphertext
  - decrypt_setting: decrypts ciphertext, raises AuthenticationError on tamper/wrong key
- Settings endpoint (app/api/settings.py):
  - GET /api/v1/settings: returns SettingsView with decrypted values, secrets masked
    (woo_app_password never returned, only woo_credentials_configured boolean)
  - PUT /api/v1/settings: encrypts all setting values AES-GCM, stores in settings table
  - Rejects woo_consumer_key/woo_consumer_secret fields
  - Validates woo_create_status (draft|publish)
- App factory (app/main.py): stores settings on app.state for dependency injection
- Unit tests (tests/unit/test_aes.py): 18 tests covering AES-1 (round-trip), AES-2
  (missing key refusal), FUND-17 (fresh nonce, tamper rejection), FUND-19
  (no plaintext fallback)

Test results: 23/23 passed (18 new AES + 5 existing health tests)

backend/app/api/settings.py

@@ -0,0 +1,223 @@
+"""Settings endpoint: GET returns masked settings, PUT stores encrypted secrets.
+
+Per OPENAPI.md §settings:
+- GET /api/v1/settings → SettingsView with secrets masked
+- PUT /api/v1/settings → update settings, encrypt secrets, return SettingsView
+
+Per AGENTS.md §4:
+- Woo credentials encrypted AES-GCM at rest
+- Ciphertext never appears in GET settings
+- Missing SLAB_AES_KEY refuses startup
+"""
+
+from datetime import datetime, timezone
+
+from fastapi import APIRouter, Depends, HTTPException, Request
+from pydantic import BaseModel, Field
+
+from app.config import Settings
+from app.db.engine import get_session
+from app.db.models import Setting
+from app.security.aes import decrypt_setting, encrypt_setting, get_aes_key
+
+router = APIRouter()
+
+
+class SettingsUpdate(BaseModel):
+    """Partial settings update payload.
+
+    Woo consumer keys are rejected.
+    """
+
+    woo_wp_username: str | None = None
+    woo_app_password: str | None = None
+    inference_base_url: str | None = None
+    inference_api_key: str | None = None
+    inference_model: str | None = None
+    inference_enabled: bool | None = None
+    content_llm_enabled: bool | None = None
+    brand_voice: str | None = None
+    geo_context: str | None = None
+    woo_create_status: str | None = Field(default=None, description="'draft' | 'publish'")
+    aspect_ratio: str | None = None
+    output_px: int | None = None
+    output_px_min: int | None = None
+    fill_target: float | None = None
+
+
+class SettingsView(BaseModel):
+    """Settings read model. Secrets masked."""
+
+    woo_base_url: str
+    woo_wp_username: str | None = None
+    woo_credentials_configured: bool = False
+    inference_base_url: str | None = None
+    inference_model: str | None = None
+    inference_enabled: bool = False
+    content_llm_enabled: bool = False
+    brand_voice: str | None = None
+    geo_context: str | None = None
+    woo_create_status: str = "draft"
+    aspect_ratio: str = "3:4"
+    output_px: int = 1600
+    output_px_min: int = 1600
+    fill_target: float = 0.80
+
+
+def _decrypt_setting_value(key_bytes: bytes, value_enc: bytes | None) -> str | None:
+    """Decrypt a setting value, returning None for null values."""
+    if value_enc is None:
+        return None
+    return decrypt_setting(key_bytes, value_enc)
+
+
+def _resolve_default(key: str, decrypted_value: str | None) -> str | None:
+    """Return decrypted value or the documented default."""
+    if decrypted_value is not None:
+        return decrypted_value
+    defaults = {
+        "woo_create_status": "draft",
+        "aspect_ratio": "3:4",
+        "output_px": 1600,
+        "output_px_min": 1600,
+        "fill_target": 0.80,
+    }
+    return defaults.get(key)
+
+
+async def _get_app_settings(request: Request) -> Settings:
+    """Dependency: retrieve application settings from app state."""
+    return request.app.state.settings
+
+
+async def _build_settings_view(app_settings: Settings = Depends(_get_app_settings)) -> SettingsView:
+    """Build a SettingsView from the database and app settings."""
+    key_bytes = get_aes_key(app_settings.aes_key)
+
+    session = await get_session()
+    rows = await session.execute("SELECT key, value_enc FROM settings")
+    all_rows = rows.all()
+    await session.close()
+
+    store = {row[0]: row[1] for row in all_rows}
+
+    woo_username = _decrypt_setting_value(key_bytes, store.get("woo_wp_username"))
+    inference_base_url = _decrypt_setting_value(key_bytes, store.get("inference_base_url"))
+    inference_model = _decrypt_setting_value(key_bytes, store.get("inference_model"))
+    inference_enabled_raw = _decrypt_setting_value(key_bytes, store.get("inference_enabled"))
+    content_llm_raw = _decrypt_setting_value(key_bytes, store.get("content_llm_enabled"))
+    brand_voice = _decrypt_setting_value(key_bytes, store.get("brand_voice"))
+    geo_context = _decrypt_setting_value(key_bytes, store.get("geo_context"))
+    woo_create_status = _resolve_default(
+        "woo_create_status",
+        _decrypt_setting_value(key_bytes, store.get("woo_create_status")),
+    )
+    aspect_ratio = _resolve_default(
+        "aspect_ratio",
+        _decrypt_setting_value(key_bytes, store.get("aspect_ratio")),
+    )
+    output_px = _resolve_default(
+        "output_px",
+        _decrypt_setting_value(key_bytes, store.get("output_px")),
+    )
+    output_px_min = _resolve_default(
+        "output_px_min",
+        _decrypt_setting_value(key_bytes, store.get("output_px_min")),
+    )
+    fill_target = _resolve_default(
+        "fill_target",
+        _decrypt_setting_value(key_bytes, store.get("fill_target")),
+    )
+
+    woo_credentials_configured = (
+        woo_username is not None
+        and _decrypt_setting_value(key_bytes, store.get("woo_app_password")) is not None
+    )
+
+    return SettingsView(
+        woo_base_url=app_settings.woo_base_url,
+        woo_wp_username=woo_username,
+        woo_credentials_configured=woo_credentials_configured,
+        inference_base_url=inference_base_url,
+        inference_model=inference_model,
+        inference_enabled=(
+            inference_enabled_raw == "true" if inference_enabled_raw is not None else False
+        ),
+        content_llm_enabled=(
+            content_llm_raw == "true" if content_llm_raw is not None else False
+        ),
+        brand_voice=brand_voice,
+        geo_context=geo_context,
+        woo_create_status=woo_create_status,
+        aspect_ratio=aspect_ratio,
+        output_px=output_px,
+        output_px_min=output_px_min,
+        fill_target=fill_target,
+    )
+
+
+@router.get("/api/v1/settings", response_model=SettingsView)
+async def get_settings(app_settings: Settings = Depends(_get_app_settings)) -> SettingsView:
+    """GET settings. Secrets (app password, inference key) are never returned.
+
+    Returns a view with decrypted non-secret values and a configured boolean
+    for credentials.
+    """
+    return await _build_settings_view(app_settings)
+
+
+@router.put("/api/v1/settings", response_model=SettingsView)
+async def update_settings(
+    update: SettingsUpdate, app_settings: Settings = Depends(_get_app_settings)
+) -> SettingsView:
+    """PUT settings. Encrypt secrets, store non-secrets. Return updated view.
+
+    Rejects consumer key fields. Returns 422 for invalid woo_create_status.
+    """
+    # Reject consumer keys
+    payload = update.model_dump(exclude_unset=True)
+    if "woo_consumer_key" in payload or "woo_consumer_secret" in payload:
+        raise HTTPException(
+            status_code=422,
+            detail="WooCommerce consumer keys are not supported. Use WordPress application passwords.",
+        )
+
+    # Validate woo_create_status
+    if update.woo_create_status is not None and update.woo_create_status not in (
+        "draft",
+        "publish",
+    ):
+        raise HTTPException(
+            status_code=422,
+            detail="woo_create_status must be 'draft' or 'publish'",
+        )
+
+    key_bytes = get_aes_key(app_settings.aes_key)
+    now = datetime.now(timezone.utc).isoformat()
+
+    session = await get_session()
+
+    for field, value in payload.items():
+        if value is None:
+            continue
+
+        ct = encrypt_setting(key_bytes, str(value))
+        setting = await session.execute(
+            "SELECT id FROM settings WHERE key = :key", {"key": field}
+        )
+        existing = setting.fetchone()
+        if existing:
+            await session.execute(
+                "UPDATE settings SET value_enc = :val, updated_at = :now WHERE key = :key",
+                {"val": ct, "now": now, "key": field},
+            )
+        else:
+            await session.execute(
+                "INSERT INTO settings (key, value_enc, updated_at) VALUES (:key, :val, :now)",
+                {"key": field, "val": ct, "now": now},
+            )
+
+    await session.commit()
+
+    # Return updated view
+    return await _build_settings_view(app_settings)