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/model/model.go

@@ -61,13 +61,15 @@ type Service struct {
 	AvailableSource string // SourceWebhook or SourcePoll; "" when AvailableRef is empty
 	AvailableAt     *time.Time
 	LastPollError   string // last registry error for the followed ref; "" when fine
-	// LastAutoDigest is the registry digest the poller last started an
-	// automatic update for; "" if never. The poller does not start another
-	// one for the same digest, so a rollback sticks until a new digest appears.
-	LastAutoDigest string
-	FirstSeenAt    time.Time
-	UpdatedAt      time.Time
-	RemovedAt      *time.Time
+	// LastTagDigest is the newest digest of the followed tag that was deployed
+	// or started: set when the poller starts an auto job and when any plain tag
+	// job succeeds (the digest the agent pinned); "" if never. The poller does
+	// not start another job for the same digest, so a rollback sticks until the
+	// registry shows a different one.
+	LastTagDigest string
+	FirstSeenAt   time.Time
+	UpdatedAt     time.Time
+	RemovedAt     *time.Time
 }
 
 // Update modes of a service.