Phase 4: backups w/ volumes, notifications, settings & users, audit page (0.4.0)

- Backup/restore: per-stack tar.gz incl. named-volume snapshots (helper
  container), upload restore with rename/overwrite/conflict detection.
- Notifications: ntfy/Discord/Slack/Gotify/generic webhooks, per-event
  subscriptions; wired into the update checker and stack lifecycle.
- Settings page: update-check interval, webhook CRUD + test, user management
  (with last-admin safeguards).
- Audit log page (searchable, paginated).
- Mobile-responsive sidebar/layout.

Multi-host agents and remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/services/settings_service.py

@@ -0,0 +1,45 @@
+"""Runtime settings stored in the DB (key/value), with env fallbacks."""
+from __future__ import annotations
+
+import json
+from typing import Any, Optional
+
+from sqlmodel import Session, select
+
+from config import settings as env_settings
+from database import engine
+from models.setting import Setting
+
+KEY_UPDATE_INTERVAL = "update_check_interval_minutes"
+
+
+def get(session: Session, key: str, default: Any = None) -> Any:
+    row = session.get(Setting, key)
+    if row is None:
+        return default
+    try:
+        return json.loads(row.value)
+    except json.JSONDecodeError:
+        return default
+
+
+def set_value(session: Session, key: str, value: Any) -> None:
+    row = session.get(Setting, key)
+    encoded = json.dumps(value)
+    if row is None:
+        session.add(Setting(key=key, value=encoded))
+    else:
+        row.value = encoded
+        session.add(row)
+    session.commit()
+
+
+def get_update_interval(session: Optional[Session] = None) -> int:
+    """Effective update-check interval in minutes (DB override or env default)."""
+    if session is None:
+        with Session(engine) as own:
+            return get_update_interval(own)
+    val = get(session, KEY_UPDATE_INTERVAL)
+    if isinstance(val, int) and val > 0:
+        return val
+    return env_settings.UPDATE_CHECK_INTERVAL_MINUTES