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/tests/integration/test_upload.py

@@ -6,7 +6,16 @@ T029 — file > MAX_UPLOAD_BYTES → 422 file_too_large
 T079 — GET /api/v1/images/{id} 404 → error envelope shape
 """
 import io
+from unittest.mock import patch
+
 import pytest
+from PIL import Image as PILImage
+
+
+def _real_jpeg(color: tuple = (100, 150, 200), size: tuple = (200, 150)) -> bytes:
+    buf = io.BytesIO()
+    PILImage.new("RGB", size, color=color).save(buf, format="JPEG")
+    return buf.getvalue()
 
 
 def _minimal_jpeg() -> bytes:
@@ -96,3 +105,51 @@ async def test_get_unknown_image_returns_404_with_envelope(client):
     body = response.json()
     assert body["code"] == "image_not_found"
     assert "detail" in body
+
+
+@pytest.mark.asyncio
+async def test_upload_returns_thumbnail_key(client):
+    data = _real_jpeg(color=(100, 150, 200))
+    response = await client.post(
+        "/api/v1/images",
+        files={"file": ("thumb_test.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert response.status_code == 201
+    body = response.json()
+    assert "thumbnail_key" in body
+    assert body["thumbnail_key"] is not None
+    assert body["thumbnail_key"].endswith("-thumb")
+
+
+@pytest.mark.asyncio
+async def test_duplicate_upload_reuses_thumbnail_key(client):
+    data = _real_jpeg(color=(200, 100, 50))
+    r1 = await client.post(
+        "/api/v1/images",
+        files={"file": ("dup.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert r1.status_code in (200, 201)
+
+    r2 = await client.post(
+        "/api/v1/images",
+        files={"file": ("dup.jpg", io.BytesIO(data), "image/jpeg")},
+    )
+    assert r2.status_code == 200
+
+    tk1 = r1.json()["thumbnail_key"]
+    tk2 = r2.json()["thumbnail_key"]
+    assert tk1 is not None
+    assert tk1 == tk2
+
+
+@pytest.mark.asyncio
+async def test_upload_succeeds_when_thumbnail_fails(client):
+    data = _real_jpeg(color=(50, 200, 150))
+    with patch("app.routers.images.generate_thumbnail", side_effect=RuntimeError("simulated")):
+        response = await client.post(
+            "/api/v1/images",
+            files={"file": ("no_thumb.jpg", io.BytesIO(data), "image/jpeg")},
+        )
+    assert response.status_code in (200, 201)
+    body = response.json()
+    assert body["thumbnail_key"] is None