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/app/repositories/image_repo.py

@@ -27,6 +27,14 @@ class ImageRepository:
         )
         return result.scalar_one_or_none()
 
+    async def get_by_short_id(self, short_id: str) -> Image | None:
+        result = await self._session.execute(
+            select(Image)
+            .where(Image.short_id == short_id)
+            .options(selectinload(Image.image_tags).selectinload(ImageTag.tag))
+        )
+        return result.scalar_one_or_none()
+
     async def create(
         self,
         *,
@@ -37,6 +45,7 @@ class ImageRepository:
         width: int,
         height: int,
         storage_key: str,
+        short_id: str,
         thumbnail_key: str | None = None,
     ) -> Image:
         image = Image(
@@ -47,6 +56,7 @@ class ImageRepository:
             width=width,
             height=height,
             storage_key=storage_key,
+            short_id=short_id,
             thumbnail_key=thumbnail_key,
         )
         self._session.add(image)