auth: replace UUID-based login with JWT credential verification

Login now requires login/email + password verified against DB via
/api/auth/login. Only approved registrations can access the app.
Signal endpoint accepts JWT Bearer tokens alongside legacy api_key auth.
Old UUID-only registration flow removed from frontend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_signal.py

@@ -78,14 +78,14 @@ async def test_signal_without_geo_success():
 
 
 @pytest.mark.asyncio
-async def test_signal_missing_user_id_returns_422():
-    """Missing user_id field must return 422."""
+async def test_signal_missing_auth_returns_401():
+    """Missing Authorization header must return 401."""
     async with make_app_client() as client:
         resp = await client.post(
             "/api/signal",
             json={"timestamp": 1742478000000},
         )
-    assert resp.status_code == 422
+    assert resp.status_code == 401
 
 
 @pytest.mark.asyncio