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_homeassistant_inbound.py

@@ -1,7 +1,5 @@
 from sqlalchemy import text
 
-import app.db as app_db
-import app.poo_db as poo_db
 from app.config import Settings, get_settings
 from app.dependencies import get_app_settings, get_homeassistant_client
 from app.main import create_app
@@ -161,42 +159,24 @@ def test_homeassistant_publish_poo_get_latest_publishes_latest_status(
     ready_location_database,
     ready_poo_database,
     auth_database,
-    monkeypatch,
 ) -> None:
-    location_engine = app_db.create_engine(
-        ready_location_database["location_url"],
-        connect_args={"check_same_thread": False},
-    )
-    location_session_local = app_db.sessionmaker(
-        bind=location_engine,
-        autoflush=False,
-        autocommit=False,
-    )
-    poo_engine = poo_db.create_engine(
-        ready_poo_database["poo_url"],
-        connect_args={"check_same_thread": False},
-    )
-    poo_session_local = poo_db.sessionmaker(
-        bind=poo_engine,
-        autoflush=False,
-        autocommit=False,
-    )
+    from fastapi.testclient import TestClient
+    from sqlalchemy import create_engine
+
+    app_url = auth_database["app_url"]
+    engine = create_engine(app_url, connect_args={"check_same_thread": False})
+
     fake_ha = _FakeHomeAssistantClient()
     settings = Settings(
         poo_sensor_entity_name="sensor.test_poo_status",
         poo_sensor_friendly_name="Poo Status",
     )
 
-    monkeypatch.setattr(app_db, "engine", location_engine)
-    monkeypatch.setattr(app_db, "SessionLocal", location_session_local)
-    monkeypatch.setattr(poo_db, "poo_engine", poo_engine)
-    monkeypatch.setattr(poo_db, "PooSessionLocal", poo_session_local)
-
     test_app = create_app()
     test_app.dependency_overrides[get_homeassistant_client] = lambda: fake_ha
     test_app.dependency_overrides[get_app_settings] = lambda: settings
 
-    with poo_engine.begin() as conn:
+    with engine.begin() as conn:
         conn.execute(
             text(
                 "INSERT INTO poo_records (timestamp, status, latitude, longitude) "
@@ -211,8 +191,6 @@ def test_homeassistant_publish_poo_get_latest_publishes_latest_status(
         )
 
     try:
-        from fastapi.testclient import TestClient
-
         with TestClient(test_app) as client:
             response = client.post(
                 "/homeassistant/publish",
@@ -233,52 +211,27 @@ def test_homeassistant_publish_poo_get_latest_publishes_latest_status(
     finally:
         test_app.dependency_overrides.clear()
         get_settings.cache_clear()
-        location_engine.dispose()
-        poo_engine.dispose()
+        engine.dispose()
 
 
 def test_homeassistant_publish_returns_internal_error_for_unknown_poo_action(
     ready_location_database,
     ready_poo_database,
     auth_database,
-    monkeypatch,
 ) -> None:
-    location_engine = app_db.create_engine(
-        ready_location_database["location_url"],
-        connect_args={"check_same_thread": False},
-    )
-    location_session_local = app_db.sessionmaker(
-        bind=location_engine,
-        autoflush=False,
-        autocommit=False,
-    )
-    poo_engine = poo_db.create_engine(
-        ready_poo_database["poo_url"],
-        connect_args={"check_same_thread": False},
-    )
-    poo_session_local = poo_db.sessionmaker(
-        bind=poo_engine,
-        autoflush=False,
-        autocommit=False,
-    )
+    from fastapi.testclient import TestClient
+
     fake_ha = _FakeHomeAssistantClient()
     settings = Settings(
         poo_sensor_entity_name="sensor.test_poo_status",
         poo_sensor_friendly_name="Poo Status",
     )
 
-    monkeypatch.setattr(app_db, "engine", location_engine)
-    monkeypatch.setattr(app_db, "SessionLocal", location_session_local)
-    monkeypatch.setattr(poo_db, "poo_engine", poo_engine)
-    monkeypatch.setattr(poo_db, "PooSessionLocal", poo_session_local)
-
     test_app = create_app()
     test_app.dependency_overrides[get_homeassistant_client] = lambda: fake_ha
     test_app.dependency_overrides[get_app_settings] = lambda: settings
 
     try:
-        from fastapi.testclient import TestClient
-
         with TestClient(test_app) as client:
             response = client.post(
                 "/homeassistant/publish",
@@ -295,8 +248,6 @@ def test_homeassistant_publish_returns_internal_error_for_unknown_poo_action(
     finally:
         test_app.dependency_overrides.clear()
         get_settings.cache_clear()
-        location_engine.dispose()
-        poo_engine.dispose()
 
 
 def test_homeassistant_publish_returns_not_implemented_for_unknown_location_action(