Feat: Substring tag search — match anywhere in tag name

Changes prefix-only LIKE to case-insensitive ILIKE with leading
wildcard so queries like "at" now match "cat", "scatter", etc.

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

api/app/repositories/tag_repo.py

@@ -88,7 +88,7 @@ class TagRepository:
 
         query = select(Tag, count_subq.label("image_count"))
         if prefix:
-            query = query.where(Tag.name.like(f"{prefix}%"))
+            query = query.where(Tag.name.ilike(f"%{prefix}%"))
         if min_count > 0:
             query = query.where(count_subq >= min_count)