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/thumbnail.py

@@ -0,0 +1,16 @@
+import contextlib
+import io
+
+from PIL import Image
+
+
+def generate_thumbnail(data: bytes, mime_type: str) -> bytes:
+    img = Image.open(io.BytesIO(data))
+    with contextlib.suppress(EOFError):
+        img.seek(0)
+    if img.mode not in ("RGB", "RGBA"):
+        img = img.convert("RGBA" if img.mode == "P" and "transparency" in img.info else "RGB")
+    img.thumbnail((400, 400), Image.LANCZOS)
+    buf = io.BytesIO()
+    img.save(buf, format="WEBP", quality=80)
+    return buf.getvalue()