feat: Add API key authentication for backup tool

- Add get_api_key() and require_api_key() to auth.py
- Add generic key-value settings storage to SettingsStore
- Add test_api_key_auth to verify the implementation
- Uses secrets.compare_digest for timing-safe comparison
- Auto-generates API key on first use and stores in settings DB

backend/src/media_library_viewer_api/services/settings_store.py

@@ -1205,6 +1205,69 @@ class SettingsStore:
             )
             return int(cur.rowcount or 0)
 
+    # ------------------------------------------------------------------
+    # Generic key-value settings
+    # ------------------------------------------------------------------
+
+    def get_settings(self) -> dict[str, Any]:
+        """Return all generic settings as a dict."""
+        self.init_schema()
+        with self.connect() as conn:
+            conn.execute(
+                """
+                CREATE TABLE IF NOT EXISTS app_settings (
+                    key TEXT PRIMARY KEY,
+                    value TEXT NOT NULL,
+                    updated_at INTEGER NOT NULL
+                )
+                """
+            )
+            rows = conn.execute("SELECT key, value FROM app_settings").fetchall()
+        return {row["key"]: row["value"] for row in rows}
+
+    def get_setting(self, key: str, default: Any = None) -> Any:
+        """Return a single setting value or default."""
+        self.init_schema()
+        with self.connect() as conn:
+            conn.execute(
+                """
+                CREATE TABLE IF NOT EXISTS app_settings (
+                    key TEXT PRIMARY KEY,
+                    value TEXT NOT NULL,
+                    updated_at INTEGER NOT NULL
+                )
+                """
+            )
+            row = conn.execute(
+                "SELECT value FROM app_settings WHERE key = ?", (key,)
+            ).fetchone()
+        return row["value"] if row else default
+
+    def update_setting(self, key: str, value: str) -> None:
+        """Set a generic key-value setting."""
+        self.init_schema()
+        now = int(time.time())
+        with self.connect() as conn:
+            conn.execute(
+                """
+                CREATE TABLE IF NOT EXISTS app_settings (
+                    key TEXT PRIMARY KEY,
+                    value TEXT NOT NULL,
+                    updated_at INTEGER NOT NULL
+                )
+                """
+            )
+            conn.execute(
+                """
+                INSERT INTO app_settings (key, value, updated_at)
+                VALUES (?, ?, ?)
+                ON CONFLICT(key) DO UPDATE SET
+                    value = excluded.value,
+                    updated_at = excluded.updated_at
+                """,
+                (key, value, now),
+            )
+
 
 _store: SettingsStore | None = None