Store a target's number of delivery attempts, at least one (closes #540)

Target.MaxRetries (max_retries) becomes Target.MaxAttempts
(max_attempts), and the form field, parser and ceiling are renamed to
match. 1 is a single attempt with no circuit breaker; above 1 retries
behind the breaker up to that many attempts. Both target forms refuse
0 and less and default a blank field to 1. The target page shows the
plain number, and the README's two paragraphs on 0 against 1 become
one sentence.

Model: opus-5-5

internal/delivery/recovery_durability_test.go

@@ -66,7 +66,7 @@ func TestRecoverySkipsPendingWithSuccessfulResult(
 
 	iCreateTarget(t, s.MainDB, targetID,
 		s.WebhookID, "already-delivered",
-		database.TargetTypeLog, "", 0,
+		database.TargetTypeLog, "", 1,
 	)
 
 	event := iSeedEvent(
@@ -129,7 +129,7 @@ func TestRecoveryContinuesTheAttemptNumbering(t *testing.T) {
 
 	iCreateTarget(t, s.MainDB, targetID,
 		s.WebhookID, "numbering",
-		database.TargetTypeLog, "", 0,
+		database.TargetTypeLog, "", 1,
 	)
 
 	event := iSeedEvent(
@@ -444,10 +444,10 @@ func TestFailedResultWriteWithRetriesLeavesDeliveryRecoverable(
 				Status:   database.DeliveryStatusPending,
 				Event:    event,
 				Target: database.Target{
-					Name:       "unwritable",
-					Type:       database.TargetTypeHTTP,
-					Config:     iHTTPConfig(ts.URL),
-					MaxRetries: 3,
+					Name:        "unwritable",
+					Type:        database.TargetTypeHTTP,
+					Config:      iHTTPConfig(ts.URL),
+					MaxAttempts: 3,
 				},
 			}
 			full.ID = d.ID