feat(greenfield): rebuild SlabUploader M1-M3 on clean foundation, all gates green

Clean re-implementation of the full product (not a port of the old M3 tree):
- P0 foundation: 13-table schema + per-call sqlite factory (thread-safe),
  FastAPI app (health, JSON logging + secret redaction, CORS, startup guard).
- P1 M1 Capture: mask/sheetKeep, geometry/rect (min-area rect), measure,
  png crop/inventory/overlay, EXIF decodeOrientedBitmap.
- P2 M2 Photos: stateful capture session (batch, pick-cal, apply mask/crop
  to batch) + photos-ready lineup.
- P3 M3 Create: real backend (crypto, woo client, taxonomy sync, pricing,
  inference Call1/Call2, review, publish, settings, slabs, photos) + real FE
  review/publish/settings screens consuming the API (no hardcoded Woo ids).
- P4 gate exit: check_requirements + check_coverage both exit 0
  (137/155 IDs, 0 gaps); backend 130/130, frontend 150/150, build green.

Cite: docs/REQUIREMENTS.md (155 rows), docs/MILESTONES.md M1-M3.

backend/tests/test_photos_api.py

@@ -12,21 +12,21 @@ import backend.db as db_mod
 
 
 @pytest.fixture
-def photos_env():
+def photos_env(monkeypatch):
     tmp = tempfile.mkdtemp(prefix="photos_")
-    os.environ["SLAB_DB_PATH"] = os.path.join(tmp, "p.db")
-    os.environ["SLAB_IMAGES_DIR"] = tmp
-    os.environ["WOO_BASE_URL"] = "https://example.com"
-    os.environ["SLAB_AES_KEY"] = base64.b64encode(b"0123456789abcdef0123456789abcdef").decode()
-    db_mod._reset_for_tests()
+    monkeypatch.setenv("SLAB_DB_PATH", os.path.join(tmp, "p.db"))
+    monkeypatch.setenv("SLAB_IMAGES_DIR", tmp)
+    monkeypatch.setenv("WOO_BASE_URL", "https://example.com")
+    monkeypatch.setenv("SLAB_AES_KEY", base64.b64encode(b"0123456789abcdef0123456789abcdef").decode())
     from backend.app import app
 
-    conn = db_mod.init_db()
+    db_mod.init_db()
+
+    conn = db_mod.get_connection()
     # seed a slab
     conn.execute("INSERT INTO slabs(id, sku, status, server_rev, created_at, updated_at) VALUES ('s1','SLAB-01','calibrated',1,'t','t')")
     conn.commit()
     yield TestClient(app), conn
-    db_mod._reset_for_tests()
     shutil.rmtree(tmp, ignore_errors=True)