Rename metadata fields for clarity and specificity

Updated `metadata` field names to `media_metadata` and `notification_metadata` in the eventspace ER schema. This improves clarity by providing more context-specific naming for better understanding and maintainability.

docs/eventspace-er-schema.mermaid

@@ -197,7 +197,7 @@ erDiagram
         integer display_order
         string title
         string description
-        json metadata
+        json media_metadata
         datetime created_at
         datetime updated_at
     }
@@ -235,7 +235,7 @@ erDiagram
         string error_message
         integer retry_count
         string external_id
-        json metadata
+        json notification_metadata
         datetime created_at
         datetime updated_at
     }