fix(master): keep rollbacks of auto services until a new digest (#9)

Adds services.last_auto_digest so the poller starts one automatic update per
registry digest, and re-reads the service before starting it. Also records a
bare image's running version as tag latest, only records well-formed
digests, and shows a digest-less version unless a digest row of its tag was
first seen at or after it.

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

@@ -3,6 +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 '';
 
 -- Every version the master has seen. '' means "no tag" / "digest unknown";
 -- never NULL, so the unique constraint deduplicates.