fix(ml): skip videos in tag_and_embed and backfill

WD14 and SigLIP are image-only. The current backfill enqueues every
ImageRecord row, and tag_and_embed retries 3 times (~3 min) on each
video before giving up, blocking the serialized ml queue. Worse,
videos never get a prediction row, so every subsequent backfill
re-enqueues them.

- tag_and_embed: early return 'unsupported_format' for video
  extensions, no retry.
- backfill: filter the outerjoin query to exclude video extensions
  via VIDEO_EXTS from app.utils.image_importer.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/tasks/ml.py

@@ -5,7 +5,7 @@ import os
 import time
 
 import numpy as np
-from sqlalchemy import and_, func
+from sqlalchemy import and_, func, or_
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 
 from app.celery_app import celery
@@ -47,6 +47,11 @@ def tag_and_embed(self, image_id: int):
         log.warning(f"tag_and_embed: file missing at {image.filepath}")
         return {'status': 'file_missing'}
 
+    # WD14 + SigLIP are image-only; videos bypass inference entirely.
+    from app.utils.image_importer import VIDEO_EXTS
+    if image.filepath.lower().endswith(VIDEO_EXTS):
+        return {'status': 'unsupported_format'}
+
     try:
         t0 = time.time()
         raw = wd14.infer_filtered(image.filepath, min_any=WD14_STORE_FLOOR)
@@ -102,6 +107,11 @@ def backfill(self, batch_size: int = 50, pause_seconds: float = 0.5):
     """
     from app.ml.wd14 import MODEL_VERSION as WD14_VER
     from app.ml.siglip import MODEL_VERSION as SIGLIP_VER
+    from app.utils.image_importer import VIDEO_EXTS
+
+    video_filter = ~or_(*[
+        func.lower(ImageRecord.filepath).like(f'%{ext}') for ext in VIDEO_EXTS
+    ])
 
     enqueued_total = 0
     last_id = 0
@@ -123,6 +133,7 @@ def backfill(self, batch_size: int = 50, pause_seconds: float = 0.5):
                 ),
             )
             .filter(ImageRecord.id > last_id)
+            .filter(video_filter)
             .filter(
                 (ImageTagPrediction.image_id.is_(None))
                 | (ImageEmbedding.image_id.is_(None))