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/conftest.py

@@ -5,10 +5,8 @@ from alembic import command
 from alembic.config import Config
 from fastapi.testclient import TestClient
 from sqlalchemy import create_engine
-from sqlalchemy.orm import sessionmaker
 
-from app.auth_db import reset_auth_db_caches
-import app.db as app_db
+from app.db import reset_db_caches
 from app.config import get_settings
 from app.main import create_app
 
@@ -47,7 +45,7 @@ def test_database_urls(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
     monkeypatch.setenv("AUTH_BOOTSTRAP_PASSWORD", "test-password")
     monkeypatch.setenv("AUTH_COOKIE_SECURE_OVERRIDE", "false")
     get_settings.cache_clear()
-    reset_auth_db_caches()
+    reset_db_caches()
 
     try:
         yield {
@@ -60,7 +58,7 @@ def test_database_urls(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
         }
     finally:
         get_settings.cache_clear()
-        reset_auth_db_caches()
+        reset_db_caches()
 
 
 @pytest.fixture
@@ -79,10 +77,10 @@ def ready_poo_database(test_database_urls):
 def auth_database(test_database_urls, monkeypatch: pytest.MonkeyPatch):
     database_url = test_database_urls["app_url"]
     command.upgrade(_make_app_alembic_config(database_url), "head")
-    reset_auth_db_caches()
+    reset_db_caches()
 
     yield test_database_urls
-    reset_auth_db_caches()
+    reset_db_caches()
 
 
 @pytest.fixture
@@ -97,46 +95,20 @@ def client(app):
 
 
 @pytest.fixture
-def location_client(
-    ready_location_database,
-    ready_poo_database,
-    auth_database,
-    monkeypatch: pytest.MonkeyPatch,
-):
-    database_url = ready_location_database["location_url"]
-
-    engine = create_engine(database_url, connect_args={"check_same_thread": False})
-    session_local = sessionmaker(bind=engine, autoflush=False, autocommit=False)
-
-    monkeypatch.setattr(app_db, "engine", engine)
-    monkeypatch.setattr(app_db, "SessionLocal", session_local)
-
+def location_client(ready_location_database, ready_poo_database, auth_database):
+    app_url = auth_database["app_url"]
+    engine = create_engine(app_url, connect_args={"check_same_thread": False})
     fastapi_app = create_app()
     with TestClient(fastapi_app) as client:
         yield client, engine
-
     engine.dispose()
 
 
 @pytest.fixture
-def poo_client(
-    ready_location_database,
-    ready_poo_database,
-    auth_database,
-    monkeypatch: pytest.MonkeyPatch,
-):
-    database_url = ready_poo_database["poo_url"]
-
-    engine = create_engine(database_url, connect_args={"check_same_thread": False})
-    session_local = sessionmaker(bind=engine, autoflush=False, autocommit=False)
-
-    import app.poo_db as poo_db
-
-    monkeypatch.setattr(poo_db, "poo_engine", engine)
-    monkeypatch.setattr(poo_db, "PooSessionLocal", session_local)
-
+def poo_client(ready_location_database, ready_poo_database, auth_database):
+    app_url = auth_database["app_url"]
+    engine = create_engine(app_url, connect_args={"check_same_thread": False})
     fastapi_app = create_app()
     with TestClient(fastapi_app) as client:
         yield client, engine
-
     engine.dispose()