feat: two-stage RRF for fair wiki vs web ranking

- Merge vector+graph into single wiki source before RRF with web
- Wiki pages no longer get 2x advantage from dual retrieval
- Add vector similarity threshold (0.7 default)
- Skip synonyms in graph search to reduce noise
- Fix duplicate entity links bug in graph search

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/services/graph_service.py

@@ -1077,8 +1077,18 @@ Feel free to expand it with more details!
                 search_query,
                 {"terms": all_terms, "limit": limit}
             )
-            logger.info(f"Graph search found {len(results)} documents")
-            return results
+
+            # Deduplicate by page_id (safety net for any edge cases)
+            seen_page_ids = set()
+            unique_results = []
+            for r in results:
+                page_id = r.get("page_id")
+                if page_id and page_id not in seen_page_ids:
+                    seen_page_ids.add(page_id)
+                    unique_results.append(r)
+
+            logger.info(f"Graph search found {len(unique_results)} unique documents (raw: {len(results)})")
+            return unique_results
         except Exception as e:
             logger.error(f"Graph document search failed: {e}", exc_info=True)
             return []