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. The column is NOT NULL and a check constraint refuses a value
below 1, so every target, whatever its type, is stored with at least
1. 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; a blank field on the add target form gives 1, and on the
edit form keeps the stored value. 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/target_config_view.go

@@ -120,8 +120,8 @@ func unavailableConfigFields() []ConfigField {
 }
 
 // slackConfigFields describes a Slack target: its masked
-// webhook URL and its retry count. Only the masked URL is
-// shown; the full URL is the credential.
+// webhook URL and its number of delivery attempts. Only the
+// masked URL is shown; the full URL is the credential.
 func slackConfigFields(t *database.Target) []ConfigField {
 	cfg, err := parseSlackConfig(t.Config)
 	if err != nil {
@@ -131,7 +131,7 @@ func slackConfigFields(t *database.Target) []ConfigField {
 	return []ConfigField{{
 		Label: "Webhook URL",
 		Value: cfg.MaskedWebhookURL(),
-	}, maxRetriesField(t)}
+	}, maxAttemptsField(t)}
 }
 
 // httpConfigFields describes an HTTP target: its destination
@@ -178,23 +178,18 @@ func httpConfigFields(t *database.Target) []ConfigField {
 		})
 	}
 
-	fields = append(fields, maxRetriesField(t))
+	fields = append(fields, maxAttemptsField(t))
 
 	return fields
 }
 
-// maxRetriesField describes a target's retry count, which lives
-// on the target row rather than in its configuration blob. A
-// stored 0 makes a single attempt, so it is shown as 1.
-func maxRetriesField(t *database.Target) ConfigField {
-	attempts := strconv.Itoa(t.MaxRetries)
-	if t.MaxRetries == 0 {
-		attempts = "1 (fire-and-forget: no retries, no circuit breaker)"
-	}
-
+// maxAttemptsField describes a target's number of delivery
+// attempts, which lives on the target row rather than in its
+// configuration blob.
+func maxAttemptsField(t *database.Target) ConfigField {
 	return ConfigField{
 		Label: "Delivery attempts",
-		Value: attempts,
+		Value: strconv.Itoa(t.MaxAttempts),
 	}
 }