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; 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/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,
 	})
 }