fix(master): remember the tag's deployed digest after any tag job (#9)

Renames last_auto_digest to last_tag_digest. FinishJob sets it to the pinned
digest after any successful job whose to_ref had no digest, so a rollback
holds however the update was started.

internal/master/store/migrations/0004_polling.sql

@@ -3,7 +3,7 @@ ALTER TABLE services ADD CONSTRAINT services_available_source_check
     CHECK (available_source IN ('', 'webhook', 'poll'));
 ALTER TABLE services ADD COLUMN available_digest text NOT NULL DEFAULT '';
 ALTER TABLE services ADD COLUMN last_poll_error  text NOT NULL DEFAULT '';
-ALTER TABLE services ADD COLUMN last_auto_digest text NOT NULL DEFAULT '';
+ALTER TABLE services ADD COLUMN last_tag_digest text NOT NULL DEFAULT '';
 
 -- Every version the master has seen. '' means "no tag" / "digest unknown";
 -- never NULL, so the unique constraint deduplicates.