feat(kg): add tsvector FTS, entity deduplication, and embedding fixes

- Replace ILIKE search with PostgreSQL tsvector/GIN full-text search
  for KG entities (migration 000031)
- Add entity deduplication system with dual-threshold strategy:
  auto-merge at 0.98+ similarity with Jaro-Winkler name match,
  flag 0.90-0.98 as candidates for manual review
- Add ScanDuplicates for bulk on-demand duplicate detection
- Add MergeEntities with advisory lock and tenant-scoped relation
  re-pointing (delete-then-update to avoid ON CONFLICT on UPDATE)
- Wire dedup inline after KG extraction pipeline
- Fix BackfillKGEmbeddings: was failing silently due to
  context.Background() missing tenant_id; now runs cross-tenant
- Fix BackfillKGEmbeddings: break on error → continue with failed
  ID tracking and max consecutive error cap
- Add EmbedEntity helper; UpsertEntity now generates embeddings
  in background goroutine
- Add HTTP endpoints: POST /kg/dedup/scan, GET /kg/dedup,
  POST /kg/merge, POST /kg/dedup/dismiss
- Add web UI: Dedup dialog with Scan All button, side-by-side
  entity comparison, merge/dismiss actions
- Add Jaro-Winkler similarity algorithm with 34 unit tests
- Update IngestExtraction to return upserted entity IDs
- Bump RequiredSchemaVersion to 31

internal/http/knowledge_graph.go

@@ -42,6 +42,10 @@ func (h *KnowledgeGraphHandler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /v1/agents/{agentID}/kg/extract", h.auth(h.handleExtract))
 	mux.HandleFunc("GET /v1/agents/{agentID}/kg/stats", h.auth(h.handleStats))
 	mux.HandleFunc("GET /v1/agents/{agentID}/kg/graph", h.auth(h.handleGraph))
+	mux.HandleFunc("POST /v1/agents/{agentID}/kg/dedup/scan", h.auth(h.handleScanDuplicates))
+	mux.HandleFunc("GET /v1/agents/{agentID}/kg/dedup", h.auth(h.handleListDedupCandidates))
+	mux.HandleFunc("POST /v1/agents/{agentID}/kg/merge", h.auth(h.handleMergeEntities))
+	mux.HandleFunc("POST /v1/agents/{agentID}/kg/dedup/dismiss", h.auth(h.handleDismissCandidate))
 }
 
 func (h *KnowledgeGraphHandler) auth(next http.HandlerFunc) http.HandlerFunc {