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

backend/app/schemas.py

@@ -109,3 +109,23 @@ class DownloadLogListResponse(BaseModel):
     total: int
     page: int
     page_size: int
+
+
+class CleanupConfig(BaseModel):
+    enabled: bool = True
+    retention_minutes: int = 10080   # 7 days
+    storage_limit_pct: int = 80
+
+
+class DiskStats(BaseModel):
+    total: int
+    used: int
+    free: int
+    used_pct: float
+
+
+class CleanupStatus(BaseModel):
+    config: CleanupConfig
+    disk: DiskStats
+    last_run: str = ""
+    last_result: dict = {}