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/models/__init__.py

@@ -3,6 +3,7 @@
 from app.models.auth import AuthSession, AuthUser
 from app.models.config import AppConfigEntry
 from app.models.location import Location
+from app.models.poo import PooRecord
 from app.models.public_ip import PublicIPHistory, PublicIPState
 
 __all__ = [
@@ -10,6 +11,7 @@ __all__ = [
     "AuthSession",
     "AuthUser",
     "Location",
+    "PooRecord",
     "PublicIPHistory",
     "PublicIPState",
 ]