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/ticktick.py

@@ -7,7 +7,7 @@ from sqlalchemy.orm import Session
 from app.config import Settings
 from app.dependencies import (
     get_app_settings,
-    get_auth_db,
+    get_db,
     get_current_auth_session,
     get_ticktick_client,
 )
@@ -39,7 +39,7 @@ def start_ticktick_auth(
 @router.get("/ticktick/auth/code")
 def handle_ticktick_auth_code(
     request: Request,
-    auth_db_session: Session = Depends(get_auth_db),
+    auth_db_session: Session = Depends(get_db),
     settings: Settings = Depends(get_app_settings),
     ticktick_client: TickTickClient = Depends(get_ticktick_client),
 ) -> Response: