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.

schema/files.dbml

@@ -1,8 +1,8 @@
 Table stored_files {
   id bigserial [pk]
   guid uuid [unique, not null, default: `gen_random_uuid()`]
-  thought_id uuid [ref: > thoughts.guid]
-  project_id uuid [ref: > projects.guid]
+  thought_id bigint [ref: > thoughts.id]
+  project_id bigint [ref: > projects.id]
   name text [not null]
   media_type text [not null]
   kind text [not null, default: 'file']
@@ -21,5 +21,5 @@ Table stored_files {
 }
 
 // Cross-file refs (for relspecgo merge)
-Ref: stored_files.thought_id > thoughts.guid [delete: set null]
-Ref: stored_files.project_id > projects.guid [delete: set null]
+Ref: stored_files.thought_id > thoughts.id [delete: set null]
+Ref: stored_files.project_id > projects.id [delete: set null]