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/poo.py

@@ -1,10 +1,10 @@
 from sqlalchemy import Float, String
 from sqlalchemy.orm import Mapped, mapped_column
 
-from app.poo_db import PooBase
+from app.db import Base
 
 
-class PooRecord(PooBase):
+class PooRecord(Base):
     __tablename__ = "poo_records"
 
     timestamp: Mapped[str] = mapped_column(String, primary_key=True)