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/services/config_page.py

@@ -7,7 +7,7 @@ from typing import Any
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from app.auth_db import reset_auth_db_caches
+from app.db import reset_db_caches
 from app.config import Settings, get_settings
 from app.models.config import AppConfigEntry
 
@@ -127,7 +127,7 @@ def sync_app_hostname_from_bootstrap(session: Session, bootstrap_settings: Setti
     current_values["APP_HOSTNAME"] = bootstrap_hostname
     _persist_config_values(session, current_values)
     get_settings.cache_clear()
-    reset_auth_db_caches()
+    reset_db_caches()
 
 
 def build_runtime_settings(session: Session, bootstrap_settings: Settings) -> Settings:
@@ -184,7 +184,7 @@ def save_config_updates(session: Session, form_data: dict[str, str], bootstrap_s
     _validate_config_values(merged_values, bootstrap_settings)
     _persist_config_values(session, merged_values)
     get_settings.cache_clear()
-    reset_auth_db_caches()
+    reset_db_caches()
 
 
 def save_config_value(
@@ -199,7 +199,7 @@ def save_config_value(
     _validate_config_values(current_values, bootstrap_settings)
     _persist_config_values(session, current_values)
     get_settings.cache_clear()
-    reset_auth_db_caches()
+    reset_db_caches()
 
 
 def is_ticktick_oauth_ready(settings: Settings) -> bool: