feat: auto cleanup with retention period, storage limit, settings UI

backend/app/database.py

@@ -42,3 +42,16 @@ async def init_db():
                 await conn.execute(text(col_def))
             except Exception:
                 pass  # Column already exists
+
+        # Seed default cleanup settings (only if not already set)
+        defaults = {
+            "cleanup_enabled": "true",
+            "cleanup_retention_minutes": "10080",   # 7 days
+            "cleanup_storage_limit_pct": "80",
+            "cleanup_last_run": "",
+            "cleanup_last_result": "",
+        }
+        for k, v in defaults.items():
+            await conn.execute(text(
+                "INSERT OR IGNORE INTO app_settings (key, value, updated_at) VALUES (:k, :v, datetime('now'))"
+            ), {"k": k, "v": v})