fix: reduce vector sync scan interval to 5s for single-user service

The test_semantic_search_answer_successful_sampling test creates a note
and waits 30s for indexing, but the scanner only ran every 60s. Aligning
with the CI overlay's 5s interval ensures new notes are indexed in time.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -91,7 +91,7 @@ services:
 
       # Semantic search configuration (ADR-007, ADR-021)
       - ENABLE_SEMANTIC_SEARCH=true
-      - VECTOR_SYNC_SCAN_INTERVAL=60
+      - VECTOR_SYNC_SCAN_INTERVAL=5
       - VECTOR_SYNC_PROCESSOR_WORKERS=1
 
       #- LOG_FORMAT=json