Feat: Pre-generate WebP thumbnails on upload for faster library load

- Add Pillow dependency and thumbnail.py with generate_thumbnail() — produces
  WebP ≤400px, preserves aspect ratio, never upscales, handles GIF frame 0
- Alembic migration 002 adds nullable thumbnail_key column to images table
- Upload route generates thumbnail via asyncio.to_thread (non-blocking),
  stores at {hash}-thumb; failure is tolerated and upload succeeds with null key
- New GET /api/v1/images/{id}/thumbnail endpoint: serves WebP thumbnail or
  falls back to original for pre-feature images; ETag + immutable cache headers
- Delete route cleans up thumbnail storage object alongside original
- Library grid switches from /file to /thumbnail for all image src bindings
- 59 tests passing (46 existing + 13 new across unit, upload, serving, delete)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/app/repositories/image_repo.py

@@ -34,6 +34,7 @@ class ImageRepository:
         width: int,
         height: int,
         storage_key: str,
+        thumbnail_key: str | None = None,
     ) -> Image:
         image = Image(
             hash=hash_hex,
@@ -43,6 +44,7 @@ class ImageRepository:
             width=width,
             height=height,
             storage_key=storage_key,
+            thumbnail_key=thumbnail_key,
         )
         self._session.add(image)
         await self._session.flush()