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/public_ip.py

@@ -1,7 +1,7 @@
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.orm import Session
 
-from app.dependencies import get_auth_db, get_current_auth_session
+from app.dependencies import get_db, get_current_auth_session
 from app.schemas.public_ip import PublicIPCheckResponse
 from app.config import get_settings
 from app.services.auth import AuthenticatedSession
@@ -12,7 +12,7 @@ router = APIRouter(tags=["public-ip"])
 
 @router.get("/public-ip/check", response_model=PublicIPCheckResponse)
 def run_public_ip_check(
-    session: Session = Depends(get_auth_db),
+    session: Session = Depends(get_db),
     current_auth: AuthenticatedSession | None = Depends(get_current_auth_session),
 ) -> PublicIPCheckResponse:
     if current_auth is None: