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/model/model.go

@@ -61,9 +61,13 @@ 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
-	FirstSeenAt     time.Time
-	UpdatedAt       time.Time
-	RemovedAt       *time.Time
+	// 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
 }
 
 // Update modes of a service.