fix(migrations): version/description as class vars, not instance vars in Migration base

__init__ was setting self.version = "0000-00-00-000000" on every instantiation,
overriding the subclass class variable. All migrations were recorded in DB
with the default version instead of their own, causing duplicate key errors.

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

backend/app/migrations/migrator.py

@@ -17,10 +17,11 @@ logger = get_logger(__name__)
 
 class Migration(ABC):
     """Base class for database migrations."""
-    
+
+    version: str = "0000-00-00-000000"  # overridden by subclass as class variable
+    description: str = ""
+
     def __init__(self):
-        self.version: str = "0000-00-00-000000"  # Format: YYYY-MM-DD-HHMMSS
-        self.description: str = ""
         self.db: Optional[AsyncIOMotorDatabase] = None
     
     @abstractmethod