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/models.py

@@ -23,6 +23,7 @@ class Image(Base):
     width: Mapped[int] = mapped_column(Integer, nullable=False)
     height: Mapped[int] = mapped_column(Integer, nullable=False)
     storage_key: Mapped[str] = mapped_column(String(64), nullable=False)
+    thumbnail_key: Mapped[str | None] = mapped_column(String(70), nullable=True, default=None)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_utcnow, nullable=False)
 
     image_tags: Mapped[list["ImageTag"]] = relationship(back_populates="image", cascade="all, delete-orphan")