proto(woo): per-browser Woo creds via X-Woo-* request headers

Prototype routing so 2–3 owners can share one store URL with different WP
application passwords. Device creds live in localStorage; server uses
request override → Settings fallback and never writes overrides into the
Settings row. Inference stays on shared Settings.

See PROTOTYPE.md for UAT with two browsers and known LAN forge risk.

backend/tests/test_per_user_woo_creds.py

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+"""Prototype: per-request X-Woo-* headers build distinct WooClients without clobbering Settings."""
+
+import base64
+import os
+import shutil
+import tempfile
+
+import pytest
+from starlette.datastructures import Headers
+
+import backend.db as db_mod
+from backend.crypto import encrypt, decrypt
+from backend.woo import (
+    WooClient,
+    client_for_request,
+    request_woo_override,
+    settings_woo_creds,
+    HEADER_WOO_USERNAME,
+    HEADER_WOO_APP_PASSWORD,
+)
+
+
+@pytest.fixture
+def db_dir():
+    tmp = tempfile.mkdtemp(prefix="per_user_woo_")
+    os.environ["SLAB_DB_PATH"] = os.path.join(tmp, "t.db")
+    os.environ["SLAB_IMAGES_DIR"] = tmp
+    os.environ["WOO_BASE_URL"] = "https://store.example.com"
+    os.environ["SLAB_AES_KEY"] = base64.b64encode(b"0123456789abcdef0123456789abcdef").decode()
+    db_mod._reset_for_tests()
+    conn = db_mod.init_db()
+    conn.execute(
+        "INSERT OR REPLACE INTO settings(key, value, value_enc) VALUES ('woo_wp_username','shop-default',NULL)"
+    )
+    conn.execute(
+        "INSERT OR REPLACE INTO settings(key, value, value_enc) VALUES ('woo_app_password',NULL,?)",
+        (encrypt("shop-secret"),),
+    )
+    conn.commit()
+    yield conn
+    db_mod._reset_for_tests()
+    shutil.rmtree(tmp, ignore_errors=True)
+
+
+def _auth_pair(client: WooClient) -> tuple[str, str]:
+    raw = base64.b64decode(client._auth).decode("utf-8")
+    user, password = raw.split(":", 1)
+    return user, password
+
+
+def test_override_headers_produce_distinct_auth_without_clobbering_settings(db_dir):
+    conn = db_dir
+    before = settings_woo_creds(conn)
+    assert before[1] == "shop-default"
+    assert decrypt(before[2]) == "shop-secret"
+
+    h_a = Headers({HEADER_WOO_USERNAME: "alice", HEADER_WOO_APP_PASSWORD: "alice-app-pw"})
+    h_b = Headers({HEADER_WOO_USERNAME: "bob", HEADER_WOO_APP_PASSWORD: "bob-app-pw"})
+
+    client_a = client_for_request(conn, h_a)
+    client_b = client_for_request(conn, h_b)
+    client_fallback = client_for_request(conn, Headers({}))
+
+    assert _auth_pair(client_a) == ("alice", "alice-app-pw")
+    assert _auth_pair(client_b) == ("bob", "bob-app-pw")
+    assert _auth_pair(client_fallback) == ("shop-default", "shop-secret")
+    assert client_a._auth != client_b._auth
+
+    after = settings_woo_creds(conn)
+    assert after == before
+    assert decrypt(after[2]) == "shop-secret"
+    # Explicit: Settings row values unchanged
+    row_u = conn.execute("SELECT value FROM settings WHERE key='woo_wp_username'").fetchone()[0]
+    row_p = conn.execute("SELECT value_enc FROM settings WHERE key='woo_app_password'").fetchone()[0]
+    assert row_u == "shop-default"
+    assert decrypt(row_p) == "shop-secret"
+
+
+def test_partial_headers_fall_back_to_settings(db_dir):
+    conn = db_dir
+    # Username only — incomplete override
+    assert request_woo_override(Headers({HEADER_WOO_USERNAME: "alice"})) is None
+    client = client_for_request(conn, Headers({HEADER_WOO_USERNAME: "alice"}))
+    assert _auth_pair(client) == ("shop-default", "shop-secret")
+
+
+def test_test_woo_endpoint_uses_request_override(db_dir, monkeypatch):
+    """API: POST /settings/test-woo with X-Woo-* builds client for that user."""
+    from fastapi.testclient import TestClient
+    from backend import app as app_mod
+
+    seen: list[str] = []
+
+    def fake_sync(client, conn):
+        seen.append(_auth_pair(client)[0])
+        return {"taxonomy_anchor": "x", "taxonomy_last_sync_at": "2026-01-01T00:00:00"}
+
+    monkeypatch.setattr(app_mod.taxonomy_mod, "sync", fake_sync)
+    tc = TestClient(app_mod.app)
+    r1 = tc.post(
+        "/api/v1/settings/test-woo",
+        headers={HEADER_WOO_USERNAME: "alice", HEADER_WOO_APP_PASSWORD: "pw-a"},
+    )
+    r2 = tc.post(
+        "/api/v1/settings/test-woo",
+        headers={HEADER_WOO_USERNAME: "bob", HEADER_WOO_APP_PASSWORD: "pw-b"},
+    )
+    r3 = tc.post("/api/v1/settings/test-woo")
+    assert r1.status_code == 200 and r1.json()["ok"] is True
+    assert r2.status_code == 200 and r2.json()["ok"] is True
+    assert r3.status_code == 200 and r3.json()["ok"] is True
+    assert seen == ["alice", "bob", "shop-default"]
+    assert settings_woo_creds(db_dir)[1] == "shop-default"