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>

specs/003-upload-thumbnails/contracts/api.md

@@ -0,0 +1,90 @@
+# API Contract: Upload Thumbnails
+
+**Branch**: `003-upload-thumbnails` | **Date**: 2026-05-03
+
+---
+
+## New endpoint
+
+### `GET /api/v1/images/{image_id}/thumbnail`
+
+Returns the thumbnail content for the given image. If no thumbnail was generated
+(image pre-dates the feature or generation failed), falls back to the full-size
+original.
+
+**Path parameters**
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `image_id` | UUID | Unique identifier of the image |
+
+**Responses**
+
+#### `200 OK` — Thumbnail (or original fallback) content
+
+| Header | Value | Notes |
+|--------|-------|-------|
+| `Content-Type` | `image/webp` | Always WebP when a thumbnail exists; original `mime_type` when falling back to the original |
+| `ETag` | `"{sha256-hex}"` | Same hash as the original image — content is immutable |
+| `Cache-Control` | `public, max-age=31536000, immutable` | Safe: thumbnail content never changes |
+
+Body: raw image bytes (WebP thumbnail, or original bytes as fallback).
+
+#### `404 Not Found` — Image not found
+
+```json
+{ "detail": "Image not found", "code": "image_not_found" }
+```
+
+#### `500 Internal Server Error` — Storage retrieval failure
+
+```json
+{ "detail": "Failed to retrieve image content", "code": "storage_error" }
+```
+
+---
+
+## Changed endpoint: `POST /api/v1/images`
+
+The upload response body gains one new field:
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `thumbnail_key` | `string \| null` | S3 key of the generated thumbnail. `null` if generation failed. |
+
+All existing fields are unchanged.
+
+**Example response** (new field only shown):
+```json
+{
+  "id": "...",
+  "thumbnail_key": "abc123…-thumb",
+  ...
+}
+```
+
+---
+
+## Changed endpoint: `GET /api/v1/images` and `GET /api/v1/images/{id}`
+
+Both metadata responses gain the same `thumbnail_key` field (`string | null`).
+
+---
+
+## UI contract
+
+The Angular `ImageService` gains one new method:
+
+```
+getThumbnailUrl(id: string): string
+  → '/api/v1/images/{id}/thumbnail'
+```
+
+The `ImageRecord` interface gains:
+
+```
+thumbnail_key: string | null;
+```
+
+The library grid component uses `getThumbnailUrl(image.id)` as the `src` for
+every grid cell. The detail component continues using `getFileUrl(image.id)`.