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/db/migrations/main/001_schema.sql

@@ -80,7 +80,7 @@ CREATE TABLE targets (
     type text NOT NULL,
     active numeric DEFAULT true,
     config text,
-    max_retries integer,
+    max_attempts integer,
     PRIMARY KEY (id),
     CONSTRAINT fk_webhooks_targets
         FOREIGN KEY (webhook_id) REFERENCES webhooks (id)