fix(library-desk): fix idempotency in create_entity_mentions

Fix create_entity_mentions to correctly count only newly created relationships,
not all relationships processed by MERGE.

**Problem**: count(r) was returning ALL relationships touched by MERGE (both
created and matched), breaking idempotency tests.

**Solution**: Use temporary flag 'just_created' set only ON CREATE, filter to
those relationships, count them, then remove the flag. This ensures the count
only includes new relationships.

Now properly returns:
- N on first run (N new relationships created)
- 0 on subsequent runs (no new relationships, all already exist)

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

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

services/library-desk/src/services/graph_service.py

@@ -1158,11 +1158,16 @@ Feel free to expand it with more details!
 
         // Create MENTIONS relationship if it doesn't exist
         MERGE (d)-[r:MENTIONS]->(e)
-        ON CREATE SET r.created_at = datetime(),
+        ON CREATE SET r.just_created = true,
+                      r.created_at = datetime(),
                       r.mention_count = 1
         ON MATCH SET r.updated_at = datetime(),
                      r.mention_count = COALESCE(r.mention_count, 0) + 1
 
+        // Count only newly created relationships
+        WITH r
+        WHERE r.just_created = true
+        REMOVE r.just_created
         RETURN count(r) as relationships_created
         """