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/homeassistant.py

@@ -11,7 +11,6 @@ from app.dependencies import (
     get_app_settings,
     get_db,
     get_homeassistant_client,
-    get_poo_db,
     get_ticktick_client,
 )
 from app.integrations.homeassistant import (
@@ -36,7 +35,6 @@ INTERNAL_SERVER_ERROR_MESSAGE = "internal server error"
 async def publish_from_homeassistant(
     request: Request,
     db: Session = Depends(get_db),
-    poo_db: Session = Depends(get_poo_db),
     settings: Settings = Depends(get_app_settings),
     homeassistant_client: HomeAssistantClient = Depends(get_homeassistant_client),
     ticktick_client: TickTickClient = Depends(get_ticktick_client),
@@ -49,7 +47,7 @@ async def publish_from_homeassistant(
             db,
             envelope,
             ticktick_client=ticktick_client,
-            poo_session=poo_db,
+            poo_session=db,
             settings=settings,
             homeassistant_client=homeassistant_client,
         )