M1-T03: unify data layer, models, deps and routes onto single app DB

Collapse the three data layers into one. app/db.py now exposes a single
Base, a cached engine bound to app_database_url with SQLite WAL enabled, and
get_engine/get_session_local/reset_db_caches/get_db_session. Delete
app/auth_db.py, app/poo_db.py and app/models/base.py. All models (auth,
config, public_ip, location, poo) inherit the one Base and register on a
single metadata. Dependencies converge to a single get_db; all routes use it.

Also update the alembic env.py files (app/location/poo) and tests that
imported the removed modules so the suite stays green, and drop the obsolete
test_legacy_style_location_db test whose flow (app reading a separate location
DB) no longer exists. Location/poo Alembic chains, adopt scripts and adoption
tests remain for M1-T04; config fields remain for M1-T05.

pytest 109 passed; ruff clean (pre-existing only); WAL verified; single
Base.metadata holds all seven tables.

app/api/routes/poo.py

@@ -7,7 +7,7 @@ from pydantic import ValidationError
 from sqlalchemy.orm import Session
 
 from app.config import Settings
-from app.dependencies import get_app_settings, get_homeassistant_client, get_poo_db
+from app.dependencies import get_app_settings, get_homeassistant_client, get_db
 from app.integrations.homeassistant import HomeAssistantClient
 from app.schemas.poo import PooRecordRequest
 from app.services.poo import publish_latest_poo_status, record_poo
@@ -21,7 +21,7 @@ INTERNAL_SERVER_ERROR_MESSAGE = "internal server error"
 @router.post("/poo/record")
 async def create_poo_record(
     request: Request,
-    db: Session = Depends(get_poo_db),
+    db: Session = Depends(get_db),
     settings: Settings = Depends(get_app_settings),
     homeassistant_client: HomeAssistantClient = Depends(get_homeassistant_client),
 ) -> Response:
@@ -56,7 +56,7 @@ async def create_poo_record(
 
 @router.get("/poo/latest")
 def notify_latest_poo(
-    db: Session = Depends(get_poo_db),
+    db: Session = Depends(get_db),
     settings: Settings = Depends(get_app_settings),
     homeassistant_client: HomeAssistantClient = Depends(get_homeassistant_client),
 ) -> Response: