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/scripts/migrate_to_short_ids.py

@@ -0,0 +1,107 @@
+"""
+Migrate existing images to use short_id-based storage keys.
+
+Run after applying Alembic migration 003 (adds short_id column).
+Run before applying migration 004 (sets short_id NOT NULL).
+
+Usage:
+    python -m scripts.migrate_to_short_ids
+"""
+
+import asyncio
+import logging
+from typing import Any
+
+from sqlalchemy import select
+
+from app.database import get_session_factory
+from app.models import Image
+from app.storage.s3_backend import S3StorageBackend
+from app.utils import generate_short_id
+
+logger = logging.getLogger(__name__)
+
+
+async def migrate_image(image: Any, storage: Any, session: Any) -> bool:
+    """Migrate one image to a short_id-based key. Returns True if migrated, False if skipped."""
+    if image.short_id is not None:
+        return False
+
+    new_short_id = generate_short_id()
+    old_key = image.storage_key
+    old_thumb_key = image.thumbnail_key
+
+    try:
+        data = await storage.get(old_key)
+        await storage.put(new_short_id, data, image.mime_type)
+        # Verify copy succeeded
+        await storage.get(new_short_id)
+    except Exception as exc:
+        logger.error("Failed to copy storage object for image %s: %s", image.id, exc)
+        return False
+
+    new_thumb_key: str | None = None
+    if old_thumb_key:
+        try:
+            thumb_data = await storage.get(old_thumb_key)
+            new_thumb_key = f"{new_short_id}-thumb"
+            await storage.put(new_thumb_key, thumb_data, "image/webp")
+            await storage.get(new_thumb_key)
+        except Exception as exc:
+            logger.warning("Failed to copy thumbnail for image %s: %s", image.id, exc)
+            new_thumb_key = None
+
+    try:
+        image.short_id = new_short_id
+        image.storage_key = new_short_id
+        image.thumbnail_key = new_thumb_key
+        await session.flush()
+
+        await storage.delete(old_key)
+        if old_thumb_key and new_thumb_key:
+            await storage.delete(old_thumb_key)
+    except Exception as exc:
+        logger.error("Failed to update DB record for image %s: %s", image.id, exc)
+        return False
+
+    return True
+
+
+async def run_migration(images: list, storage: Any, session: Any) -> tuple[int, int, int]:
+    """Process a list of images. Returns (migrated, skipped, failed) counts."""
+    migrated = skipped = failed = 0
+    for image in images:
+        if image.short_id is not None:
+            skipped += 1
+            continue
+        try:
+            success = await migrate_image(image, storage, session)
+            if success:
+                migrated += 1
+            else:
+                failed += 1
+        except Exception as exc:
+            logger.error("Unexpected error migrating image %s: %s", image.id, exc)
+            failed += 1
+
+    return migrated, skipped, failed
+
+
+async def main() -> None:
+    logging.basicConfig(level=logging.INFO)
+
+    storage = S3StorageBackend()
+
+    async with get_session_factory()() as session:
+        result = await session.execute(select(Image).where(Image.short_id.is_(None)))
+        images = list(result.scalars().all())
+        logger.info("Found %d images to migrate", len(images))
+
+        migrated, skipped, failed = await run_migration(images, storage, session)
+        await session.commit()
+
+    print(f"Migrated: {migrated}, Skipped: {skipped}, Failed: {failed}")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())