Add local backup retention: keep-local toggle, download, delete-local

Lets operators opt into keeping the local archive after upload (global
keep_local_backups setting), pull a remote archive back down into the
Proxmox dump directory for manual restore, and drop a locally-retained
copy without touching the remote object. Consolidates deletion so
retention, job-cascade delete, manual delete, and crash-recovery resume
all clean up local copies alongside remote ones.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/main.py

@@ -15,6 +15,8 @@ from .backup_service import (
     dashboard,
     delete_backup_manually,
     delete_job_with_backups,
+    delete_local_copy,
+    download_backup_locally,
     get_backup,
     list_backups,
     process_backup,
@@ -40,6 +42,7 @@ class SettingsPayload(BaseModel):
     default_retention_value: int = 30
     max_concurrent_backups: int = 1
     timezone: str = "UTC"
+    keep_local_backups: bool = False
     setup_complete: bool = False
 
 
@@ -252,6 +255,28 @@ async def delete_backup(backup_id: str):
         raise HTTPException(409, str(exc)) from exc
 
 
+@app.post("/api/backups/{backup_id}/download")
+async def download_backup(backup_id: str):
+    try:
+        return await download_backup_locally(backup_id)
+    except KeyError as exc:
+        raise HTTPException(404, str(exc)) from exc
+    except RuntimeError as exc:
+        raise HTTPException(409, str(exc)) from exc
+    except Exception as exc:
+        raise HTTPException(502, f"Download failed: {exc}") from exc
+
+
+@app.delete("/api/backups/{backup_id}/local")
+async def delete_backup_local_copy(backup_id: str):
+    try:
+        return await delete_local_copy(backup_id)
+    except KeyError as exc:
+        raise HTTPException(404, str(exc)) from exc
+    except RuntimeError as exc:
+        raise HTTPException(409, str(exc)) from exc
+
+
 if (STATIC_DIR / "assets").exists():
     app.mount("/assets", StaticFiles(directory=STATIC_DIR / "assets"), name="assets")