refactor(store,tools): migrate IDs from UUID to bigserial int64

All internal entity lookups now use bigserial primary keys (int64) while
GUIDs are retained for external/public identification. Updated store
functions (TouchProject, UpdateThoughtMetadata, AddThoughtAttachment) to
query by id instead of guid, added GetThoughtByID, changed semanticSearch
and all tool helpers to use *int64 project IDs, and updated retry/backfill
workers to use int64 thought IDs throughout.

internal/tools/retrieval.go

@@ -23,7 +23,7 @@ func semanticSearch(
 	query string,
 	limit int,
 	threshold float64,
-	projectID *uuid.UUID,
+	projectID *int64,
 	excludeID *uuid.UUID,
 ) ([]thoughttypes.SearchResult, error) {
 	model := embeddings.PrimaryModel()