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_location.py

@@ -5,18 +5,14 @@ import sqlite3
 import pytest
 from alembic import command
 from alembic.config import Config
-from sqlalchemy import create_engine, text
-from sqlalchemy.orm import sessionmaker
+from sqlalchemy import text
 
-import app.db as app_db
-from app.main import create_app
 from scripts.location_db_adopt import (
     EXPECTED_USER_VERSION,
     LOCATION_BASELINE_REVISION,
     LocationDatabaseAdoptionError,
     adopt_or_initialize_location_db,
 )
-from tests.conftest import _make_app_alembic_config, _make_poo_alembic_config
 
 
 def _make_alembic_config(database_url: str) -> Config:
@@ -197,66 +193,6 @@ def test_location_record_endpoint_defaults_invalid_altitude_to_zero(location_cli
     assert row.altitude == pytest.approx(0.0)
 
 
-def test_legacy_style_location_db_can_be_stamped_and_adopted(
-    test_database_urls, monkeypatch: pytest.MonkeyPatch
-) -> None:
-    app_database_url = test_database_urls["app_url"]
-    database_path = test_database_urls["location_path"]
-    database_url = test_database_urls["location_url"]
-    poo_database_url = test_database_urls["poo_url"]
-
-    conn = sqlite3.connect(database_path)
-    conn.execute(
-        """
-        CREATE TABLE location (
-          person TEXT NOT NULL,
-          datetime TEXT NOT NULL,
-          latitude REAL NOT NULL,
-          longitude REAL NOT NULL,
-          altitude REAL,
-          PRIMARY KEY (person, datetime)
-        )
-        """
-    )
-    conn.execute("PRAGMA user_version = 2")
-    conn.commit()
-    conn.close()
-
-    command.upgrade(_make_app_alembic_config(app_database_url), "head")
-    command.stamp(_make_alembic_config(database_url), LOCATION_BASELINE_REVISION)
-    command.upgrade(_make_poo_alembic_config(poo_database_url), "head")
-
-    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)
-
-    from fastapi.testclient import TestClient
-
-    fastapi_app = create_app()
-    with TestClient(fastapi_app) as client:
-        response = client.post(
-            "/location/record",
-            json={
-                "person": "legacy-user",
-                "latitude": "12.3",
-                "longitude": "45.6",
-                "altitude": "7.8",
-            },
-        )
-
-    assert response.status_code == 200
-
-    with engine.connect() as db_conn:
-        revision = db_conn.execute(text("SELECT version_num FROM alembic_version")).scalar_one()
-        row_count = db_conn.execute(text("SELECT COUNT(*) FROM location")).scalar_one()
-
-    assert revision == LOCATION_BASELINE_REVISION
-    assert row_count == 1
-
-    engine.dispose()
-
-
 def test_location_db_adoption_initializes_new_db(tmp_path: Path) -> None:
     database_path = tmp_path / "new_location.db"
     result = adopt_or_initialize_location_db(f"sqlite:///{database_path}")