Add suitbuilder backend improvements and SSE streaming fix

- Add dedicated streaming proxy endpoint for real-time suitbuilder SSE updates
- Implement stable sorting with character_name and name tiebreakers for deterministic results
- Refactor locked items to locked slots supporting set_id and spell constraints
- Add Mag-SuitBuilder style branch pruning tracking variables
- Enhance search with phase updates and detailed progress logging
- Update design document with SSE streaming proxy fix details

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

inventory-service/main.py

@@ -2702,7 +2702,7 @@ async def search_items(
 
         # Handle NULLS for optional fields
         nulls_clause = "NULLS LAST" if sort_direction == "ASC" else "NULLS FIRST"
-        query_parts.append(f"ORDER BY {sort_field} {sort_direction} {nulls_clause}")
+        query_parts.append(f"ORDER BY {sort_field} {sort_direction} {nulls_clause}, character_name, db_item_id")
         
         # Add pagination
         offset = (page - 1) * limit