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/dependencies.py

@@ -3,30 +3,20 @@ from collections.abc import Generator
 from fastapi import Depends, Request
 from sqlalchemy.orm import Session
 
-from app.auth_db import get_auth_db_session
 from app.config import Settings, get_settings
 from app.db import get_db_session
 from app.integrations.homeassistant import HomeAssistantClient
 from app.integrations.ticktick import TickTickClient
-from app.poo_db import get_poo_db_session
 from app.services.auth import AuthenticatedSession, get_authenticated_session
 from app.services.config_page import build_runtime_settings
 
 
-def get_auth_db() -> Generator[Session, None, None]:
-    yield from get_auth_db_session()
-
-
-def get_app_settings(session: Session = Depends(get_auth_db)) -> Settings:
-    return build_runtime_settings(session, get_settings())
-
-
 def get_db() -> Generator[Session, None, None]:
     yield from get_db_session()
 
 
-def get_poo_db() -> Generator[Session, None, None]:
-    yield from get_poo_db_session()
+def get_app_settings(session: Session = Depends(get_db)) -> Settings:
+    return build_runtime_settings(session, get_settings())
 
 
 def get_homeassistant_client(settings: Settings = Depends(get_app_settings)) -> HomeAssistantClient:
@@ -39,7 +29,7 @@ def get_ticktick_client(settings: Settings = Depends(get_app_settings)) -> TickT
 
 def get_current_auth_session(
     request: Request,
-    session: Session = Depends(get_auth_db),
+    session: Session = Depends(get_db),
     settings: Settings = Depends(get_app_settings),
 ) -> AuthenticatedSession | None:
     raw_token = request.cookies.get(settings.auth_session_cookie_name)