fix: review fixes for webhook and auto mode (#8)

Clear available_* only when the deployed ref equals it, drop the unused
poll source, stop leaking internal errors to CI, use the request-scoped
logger, validate X-Request-ID, re-read jobs once after the timeout,
report refused when no target ref exists, and tidy UI, deploy and docs.

internal/master/model/model.go

@@ -56,7 +56,7 @@ type Service struct {
 	// AvailableRef is the newest ref a webhook or poll reported for this
 	// service; "" if none, or if a later job already deployed it.
 	AvailableRef    string
-	AvailableSource string // SourceWebhook or SourcePoll; "" when AvailableRef is empty
+	AvailableSource string // SourceWebhook; "" when AvailableRef is empty
 	AvailableAt     *time.Time
 	FirstSeenAt     time.Time
 	UpdatedAt       time.Time
@@ -72,13 +72,12 @@ const (
 // Sources of an available ref.
 const (
 	SourceWebhook = "webhook"
-	SourcePoll    = "poll"
 )
 
 // Available is a newer ref reported for a service.
 type Available struct {
 	Ref    string
-	Source string // SourceWebhook or SourcePoll
+	Source string // SourceWebhook
 	At     time.Time
 }