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.

tests/test_auth.py

@@ -4,7 +4,7 @@ from pathlib import Path
 
 from fastapi.testclient import TestClient
 
-from app.auth_db import reset_auth_db_caches
+from app.db import reset_db_caches
 from app.config import get_settings
 from app.main import create_app
 
@@ -205,7 +205,7 @@ def test_config_page_shows_ticktick_oauth_link_when_ticktick_is_configured(
     monkeypatch.setenv("TICKTICK_CLIENT_ID", "ticktick-client-id")
     monkeypatch.setenv("TICKTICK_CLIENT_SECRET", "ticktick-client-secret")
     get_settings.cache_clear()
-    reset_auth_db_caches()
+    reset_db_caches()
 
     with TestClient(create_app()) as client:
         login_page = client.get("/login")