feat(maintenance): daily janitor for orphaned .part/.partial staging files

Downloads/imports stage into <name>.part / <name>.partial then os.replace() into
place, so a kill mid-write leaves a discardable temp — never a corrupt final.
cleanup_orphaned_temp_files sweeps ones left behind under the images root, only
older than 6h so an in-flight download's staging file is never removed. Daily beat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/celery_app.py

@@ -115,6 +115,11 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.maintenance.cleanup_old_tasks",
                 "schedule": 86400.0,  # daily
             },
+            "cleanup-orphaned-temp-files": {
+                "task": "backend.app.tasks.maintenance.cleanup_orphaned_temp_files",
+                "schedule": 86400.0,  # daily — sweep .part/.partial left by a
+                # download/import killed mid-write (graceful-shutdown fallout)
+            },
             "train-heads-nightly": {
                 "task": "backend.app.tasks.ml.scheduled_train_heads",
                 "schedule": 86400.0,  # passive cadence; manual retrain stays available

backend/app/tasks/maintenance.py

@@ -79,6 +79,14 @@ VERIFY_PAGE = 200
 FFPROBE_TIMEOUT_SECONDS = 10
 TASK_RUN_KEEP_OK_SECONDS = 24 * 3600       # 24 h
 TASK_RUN_KEEP_FAILURE_SECONDS = 7 * 24 * 3600  # 7 days
+# Orphaned staging files: downloads/imports stage into <name>.part / <name>.partial
+# then os.replace() into place (importer / external_fetch / native_ingest_common /
+# attachment_store), so a kill mid-write leaves a discardable temp, never a corrupt
+# final. cleanup_orphaned_temp_files sweeps ones left behind; the min-age guard
+# keeps it from deleting an in-flight download's staging file mid-write.
+IMAGES_ROOT = Path("/images")
+TEMP_STAGING_SUFFIXES = (".part", ".partial")
+ORPHAN_TEMP_MIN_AGE_HOURS = 6
 
 # Audit 2026-06-02: per-entity recovery sweep thresholds. Each must be
 # > the entity's longest legitimate runtime (its task's time_limit + a
@@ -339,6 +347,34 @@ def cleanup_old_tasks() -> int:
         return result.rowcount or 0
 
 
+@celery.task(name="backend.app.tasks.maintenance.cleanup_orphaned_temp_files")
+def cleanup_orphaned_temp_files() -> int:
+    """Delete orphaned .part/.partial staging files under the images root, left by
+    a download/import killed mid-write. Only removes files older than
+    ORPHAN_TEMP_MIN_AGE_HOURS so an in-flight download's staging file is never
+    pulled out from under it. Returns the count removed."""
+    if not IMAGES_ROOT.is_dir():
+        return 0
+    cutoff = datetime.now(UTC).timestamp() - ORPHAN_TEMP_MIN_AGE_HOURS * 3600
+    removed = 0
+    for path in IMAGES_ROOT.rglob("*"):
+        if path.suffix not in TEMP_STAGING_SUFFIXES or not path.is_file():
+            continue
+        try:
+            if path.stat().st_mtime >= cutoff:
+                continue          # too fresh — may be an active download
+            path.unlink()
+            removed += 1
+        except OSError as exc:
+            log.warning("cleanup_orphaned_temp_files: %s: %s", path, exc)
+    if removed:
+        log.info(
+            "cleanup_orphaned_temp_files: removed %d orphaned staging file(s)",
+            removed,
+        )
+    return removed
+
+
 @celery.task(name="backend.app.tasks.maintenance.recover_stalled_task_runs")
 def recover_stalled_task_runs() -> int:
     """Flip task_run rows stuck in 'running' past their queue-specific