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>

ui/src/app/services/image.service.ts

@@ -11,6 +11,7 @@ export interface ImageRecord {
   width: number;
   height: number;
   storage_key: string;
+  thumbnail_key: string | null;
   created_at: string;
   tags: string[];
   duplicate?: boolean;
@@ -54,6 +55,10 @@ export class ImageService {
     return `${this.base}/images/${id}/file`;
   }
 
+  getThumbnailUrl(id: string): string {
+    return `${this.base}/images/${id}/thumbnail`;
+  }
+
   updateTags(id: string, tags: string[]): Observable<ImageRecord> {
     return this.http.patch<ImageRecord>(`${this.base}/images/${id}/tags`, { tags });
   }