tune(artwork): drop backoff base from 5m to 15s

The exponential retry (base × 4^attempts, cap 48h) started at 5 minutes, so a
single transient failure — a timeout under load, an external blip — parked a
cover for 5 minutes even though a retry seconds later would have resolved it.
Start at 15s instead: transient failures recover almost immediately (15s → 1m
→ 4m → 16m …), while persistent failures still escalate to the 48h cap (now at
the 8th attempt instead of the 5th).

core/artwork/worker.go

@@ -22,7 +22,7 @@ import (
 
 const (
 	workerPollInterval = 5 * time.Second
-	backoffBase        = 5 * time.Minute
+	backoffBase        = 15 * time.Second
 	backoffCap         = 48 * time.Hour
 	breakerThreshold   = 5
 	breakerProbeAfter  = time.Minute

core/artwork/worker_test.go

@@ -565,13 +565,15 @@ var _ = Describe("backoff", func() {
 			attempts int
 			want     time.Duration
 		}{
-			{0, 5 * time.Minute},
-			{1, 20 * time.Minute},
-			{2, 80 * time.Minute},
-			{3, 320 * time.Minute},
-			{4, 1280 * time.Minute},
-			{5, 48 * time.Hour},
-			{6, 48 * time.Hour},
+			{0, 15 * time.Second},
+			{1, 60 * time.Second},
+			{2, 4 * time.Minute},
+			{3, 16 * time.Minute},
+			{4, 64 * time.Minute},
+			{5, 256 * time.Minute},
+			{6, 1024 * time.Minute},
+			{7, 48 * time.Hour},
+			{8, 48 * time.Hour},
 		} {
 			Expect(backoffFor(c.attempts, 0)).To(Equal(c.want), "attempt %d", c.attempts)
 		}
@@ -584,8 +586,8 @@ var _ = Describe("backoff", func() {
 	})
 
 	It("keeps random jitter within +/-20%", func() {
-		lo := time.Duration(float64(320*time.Minute) * 0.8)
-		hi := time.Duration(float64(320*time.Minute) * 1.2)
+		lo := time.Duration(float64(16*time.Minute) * 0.8)
+		hi := time.Duration(float64(16*time.Minute) * 1.2)
 		for range 200 {
 			d := backoff(3)
 			Expect(d).To(BeNumerically(">=", lo))