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

static/js/app.js

@@ -139,7 +139,7 @@ document.addEventListener("alpine:init", function () {
             headers: "",
             timeout: "",
             forwardQuery: false,
-            maxRetries: "",
+            maxAttempts: "",
             expiry: "",
             rotation: "",
             init() {
@@ -153,7 +153,7 @@ document.addEventListener("alpine:init", function () {
                 this.timeout = refused.timeout;
                 this.forwardQuery =
                     this.$root.hasAttribute("data-forward-query");
-                this.maxRetries = refused.maxRetries;
+                this.maxAttempts = refused.maxAttempts;
                 this.expiry = refused.expiry;
                 this.rotation = refused.rotation;
             },
@@ -173,7 +173,7 @@ document.addEventListener("alpine:init", function () {
                 this.headers = "";
                 this.timeout = "";
                 this.forwardQuery = false;
-                this.maxRetries = "";
+                this.maxAttempts = "";
                 this.expiry = "";
                 this.rotation = "";
                 this.$refs.form.reset();