Feat: Replace UUID image identifiers with 8-character base62 short IDs

Short IDs become the canonical identifier in URLs (/i/:short_id),
MinIO/R2 storage keys, and all API responses. Hash-based deduplication
is preserved. Includes two-phase Alembic migration (003 adds nullable
column, 004 enforces NOT NULL) with a backfill script to copy storage
objects and populate short_id for existing images.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/tests/unit/test_migration.py

@@ -0,0 +1,110 @@
+"""Unit tests for migrate_to_short_ids script logic."""
+
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+@pytest.fixture
+def mock_image_null_short_id():
+    img = MagicMock()
+    img.id = "img-uuid-1"
+    img.short_id = None
+    img.storage_key = "oldhashkey1234567890"
+    img.thumbnail_key = "oldhashkey1234567890-thumb"
+    img.mime_type = "image/jpeg"
+    return img
+
+
+@pytest.fixture
+def mock_image_with_short_id():
+    img = MagicMock()
+    img.id = "img-uuid-2"
+    img.short_id = "AbCd1234"
+    img.storage_key = "AbCd1234"
+    img.thumbnail_key = "AbCd1234-thumb"
+    img.mime_type = "image/jpeg"
+    return img
+
+
+@pytest.mark.asyncio
+async def test_migrate_processes_image_without_short_id(mock_image_null_short_id):
+    """Images with short_id IS NULL are processed: storage copied, DB updated, old keys deleted."""
+    from scripts.migrate_to_short_ids import migrate_image
+
+    storage = MagicMock()
+    storage.get = AsyncMock(return_value=b"imagedata")
+    storage.put = AsyncMock()
+    storage.delete = AsyncMock()
+
+    session = MagicMock()
+    session.execute = AsyncMock()
+    session.flush = AsyncMock()
+
+    old_key = mock_image_null_short_id.storage_key
+    new_short_id = "NewSh123"
+    with patch("scripts.migrate_to_short_ids.generate_short_id", return_value=new_short_id):
+        result = await migrate_image(mock_image_null_short_id, storage, session)
+
+    assert result is True
+    storage.put.assert_any_call(new_short_id, b"imagedata", "image/jpeg")
+    storage.delete.assert_any_call(old_key)
+
+
+@pytest.mark.asyncio
+async def test_migrate_skips_image_with_short_id(mock_image_with_short_id):
+    """Images that already have a short_id are skipped."""
+    from scripts.migrate_to_short_ids import migrate_image
+
+    storage = MagicMock()
+    session = MagicMock()
+
+    result = await migrate_image(mock_image_with_short_id, storage, session)
+
+    assert result is False
+    storage.get.assert_not_called() if hasattr(storage.get, "assert_not_called") else None
+
+
+@pytest.mark.asyncio
+async def test_migrate_continues_on_storage_error(mock_image_null_short_id):
+    """If storage copy fails, error is logged and migrate_image returns False without aborting."""
+    from scripts.migrate_to_short_ids import migrate_image
+
+    storage = MagicMock()
+    storage.get = AsyncMock(side_effect=Exception("storage read error"))
+    storage.put = AsyncMock()
+    storage.delete = AsyncMock()
+
+    session = MagicMock()
+    session.execute = AsyncMock()
+    session.flush = AsyncMock()
+
+    with patch("scripts.migrate_to_short_ids.generate_short_id", return_value="ErrSh123"):
+        result = await migrate_image(mock_image_null_short_id, storage, session)
+
+    assert result is False
+    storage.put.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_migrate_summary_counts(mock_image_null_short_id, mock_image_with_short_id):
+    """run_migration reports correct migrated and skipped counts."""
+    from scripts.migrate_to_short_ids import run_migration
+
+    storage = MagicMock()
+    storage.get = AsyncMock(return_value=b"data")
+    storage.put = AsyncMock()
+    storage.delete = AsyncMock()
+
+    session = MagicMock()
+    session.execute = AsyncMock()
+    session.flush = AsyncMock()
+
+    images = [mock_image_null_short_id, mock_image_with_short_id]
+
+    with patch("scripts.migrate_to_short_ids.generate_short_id", return_value="NewSh999"):
+        migrated, skipped, failed = await run_migration(images, storage, session)
+
+    assert migrated == 1
+    assert skipped == 1
+    assert failed == 0