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/db.py

@@ -1,6 +1,8 @@
 from collections.abc import Generator
+from functools import lru_cache
 
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, event
+from sqlalchemy.engine import Engine
 from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
 
 from app.config import get_settings
@@ -10,18 +12,49 @@ class Base(DeclarativeBase):
     pass
 
 
-settings = get_settings()
+def _build_connect_args(database_url: str) -> dict[str, object]:
+    connect_args: dict[str, object] = {}
+    if database_url.startswith("sqlite"):
+        connect_args["check_same_thread"] = False
+    return connect_args
 
-connect_args: dict[str, object] = {}
-if settings.location_database_url.startswith("sqlite"):
-    connect_args["check_same_thread"] = False
 
-engine = create_engine(settings.location_database_url, connect_args=connect_args)
-SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False, class_=Session)
+@lru_cache
+def _get_engine(database_url: str) -> Engine:
+    engine = create_engine(database_url, connect_args=_build_connect_args(database_url))
+    if database_url.startswith("sqlite"):
+
+        @event.listens_for(engine, "connect")
+        def _enable_sqlite_wal(dbapi_connection, _connection_record):
+            cursor = dbapi_connection.cursor()
+            cursor.execute("PRAGMA journal_mode=WAL")
+            cursor.close()
+
+    return engine
+
+
+@lru_cache
+def _get_session_local(database_url: str) -> sessionmaker:
+    engine = _get_engine(database_url)
+    return sessionmaker(bind=engine, autoflush=False, autocommit=False, class_=Session)
+
+
+def get_engine() -> Engine:
+    return _get_engine(get_settings().app_database_url)
+
+
+def get_session_local() -> sessionmaker:
+    return _get_session_local(get_settings().app_database_url)
+
+
+def reset_db_caches() -> None:
+    _get_session_local.cache_clear()
+    _get_engine.cache_clear()
 
 
 def get_db_session() -> Generator[Session, None, None]:
-    session = SessionLocal()
+    session_local = get_session_local()
+    session = session_local()
     try:
         yield session
     finally: