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/upload/upload.component.spec.ts

@@ -11,7 +11,8 @@ describe('UploadComponent', () => {
   let component: UploadComponent;
 
   function makeImageService(overrides: Partial<ImageService> = {}): jasmine.SpyObj<ImageService> {
-    return jasmine.createSpyObj<ImageService>('ImageService', { upload: of({} as any), ...overrides });
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    return jasmine.createSpyObj<ImageService>('ImageService', { upload: of({} as any), ...overrides } as any);
   }
 
   beforeEach(async () => {