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.

alembic_app/env.py

@@ -3,11 +3,13 @@ from logging.config import fileConfig
 from alembic import context
 from sqlalchemy import engine_from_config, pool
 
-from app.auth_db import AuthBase
 from app.config import get_settings
+from app.db import Base
 from app.models.config import AppConfigEntry  # noqa: F401
 from app.models.auth import AuthSession, AuthUser  # noqa: F401
 from app.models.public_ip import PublicIPHistory, PublicIPState  # noqa: F401
+from app.models.location import Location  # noqa: F401
+from app.models.poo import PooRecord  # noqa: F401
 
 config = context.config
 
@@ -19,7 +21,7 @@ configured_url = config.get_main_option("sqlalchemy.url")
 if not configured_url or configured_url == "sqlite:///./data/app.db":
     config.set_main_option("sqlalchemy.url", settings.app_database_url)
 
-target_metadata = AuthBase.metadata
+target_metadata = Base.metadata
 
 
 def run_migrations_offline() -> None: