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/main.py

@@ -10,7 +10,7 @@ from sqlalchemy.orm import Session
 from app import models  # noqa: F401
 from app.api.routes.auth import router as auth_router
 from app.api.routes import pages, status
-import app.auth_db as auth_db
+from app.db import get_session_local
 from app.api.routes.homeassistant import router as homeassistant_router
 from app.api.routes.location import router as location_router
 from app.api.routes.poo import router as poo_router
@@ -26,7 +26,7 @@ from scripts.poo_db_adopt import PooDatabaseAdoptionError, validate_poo_runtime_
 
 
 def _run_scheduled_public_ip_check() -> None:
-    session_local = auth_db.get_auth_session_local()
+    session_local = get_session_local()
     session: Session = session_local()
     try:
         check_public_ipv4_and_notify(session, bootstrap_settings=get_settings())
@@ -35,7 +35,7 @@ def _run_scheduled_public_ip_check() -> None:
 
 
 def ensure_auth_db_ready() -> None:
-    session_local = auth_db.get_auth_session_local()
+    session_local = get_session_local()
     session: Session = session_local()
     try:
         validate_app_runtime_db(get_settings().app_database_url)