feat(fc2a): add Celery tasks — scan_directory, import_media_file, generate_thumbnail

scan_directory walks ImportSettings.import_scan_path, creates an
ImportBatch, enumerates supported files into ImportTasks, and enqueues
import_media_file per task. import_media_file moves the task through
its state machine (pending → queued → processing → complete/skipped/failed),
updates ImportBatch counters atomically (UPDATE ... SET col = col + 1),
enqueues a thumbnail task on success, and marks the batch complete when
the last task drains.

generate_thumbnail runs on its own queue (thumbnail) so big imports
don't starve thumbnail throughput; failure here is logged and does not
fail the import.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/tasks/thumbnail.py

@@ -0,0 +1,50 @@
+"""generate_thumbnail task: PIL/ffmpeg thumbnail generation on the thumbnail queue.
+
+Lives separately from import_file because thumbnails can be regenerated en
+masse (FC-2c adds the 'regenerate all' admin action) and they're CPU-bound
+so they deserve their own queue lane.
+"""
+
+from pathlib import Path
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from ..celery_app import celery
+from ..config import get_config
+from ..models import ImageRecord
+from ..services.importer import is_video
+from ..services.thumbnailer import Thumbnailer
+
+
+IMAGES_ROOT = Path("/images")
+
+
+def _sync_session_factory():
+    cfg = get_config()
+    engine = create_engine(cfg.database_url_sync, future=True, pool_pre_ping=True)
+    return sessionmaker(engine, expire_on_commit=False)
+
+
+@celery.task(name="backend.app.tasks.thumbnail.generate_thumbnail", bind=True)
+def generate_thumbnail(self, image_id: int) -> dict:
+    SessionLocal = _sync_session_factory()
+    with SessionLocal() as session:
+        record = session.get(ImageRecord, image_id)
+        if record is None:
+            return {"status": "missing", "image_id": image_id}
+
+        thumbnailer = Thumbnailer(images_root=IMAGES_ROOT)
+        source = Path(record.path)
+        try:
+            if is_video(source):
+                result = thumbnailer.generate_video_thumbnail(source, record.sha256)
+            else:
+                result = thumbnailer.generate_image_thumbnail(source, record.sha256)
+        except Exception as exc:  # pragma: no cover — thumbnail failure is non-fatal
+            return {"status": "failed", "image_id": image_id, "error": f"{type(exc).__name__}: {exc}"}
+
+        record.thumbnail_path = str(result.path)
+        session.add(record)
+        session.commit()
+        return {"status": "ok", "image_id": image_id, "path": str(result.path)}