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/library/library.component.ts

@@ -48,7 +48,7 @@ import { TagService } from '../services/tag.service';
           class="image-card"
           (click)="router.navigate(['/images', img.id])"
         >
-          <img [src]="imageService.getFileUrl(img.id)" [alt]="img.filename" loading="lazy" />
+          <img [src]="imageService.getThumbnailUrl(img.id)" [alt]="img.filename" loading="lazy" />
           <div class="tag-row">
             <span *ngFor="let tag of img.tags" class="chip small">{{ tag }}</span>
           </div>