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/database/schema_test.go

@@ -87,13 +87,13 @@ func TestMainSchemaHoldsEveryModel(t *testing.T) {
 		Active:      true,
 	})
 	assertRoundTrip(t, db, &database.Target{
-		BaseModel:  newBaseModel(),
-		WebhookID:  uuid.New().String(),
-		Name:       "target",
-		Type:       database.TargetTypeHTTP,
-		Active:     true,
-		Config:     `{"url":"https://example.com/"}`,
-		MaxRetries: 3,
+		BaseModel:   newBaseModel(),
+		WebhookID:   uuid.New().String(),
+		Name:        "target",
+		Type:        database.TargetTypeHTTP,
+		Active:      true,
+		Config:      `{"url":"https://example.com/"}`,
+		MaxAttempts: 3,
 	})
 }