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/db/migrations/main/001_schema.sql

@@ -80,10 +80,11 @@ CREATE TABLE targets (
     type text NOT NULL,
     active numeric DEFAULT true,
     config text,
-    max_retries integer,
+    max_attempts integer NOT NULL,
     PRIMARY KEY (id),
     CONSTRAINT fk_webhooks_targets
-        FOREIGN KEY (webhook_id) REFERENCES webhooks (id)
+        FOREIGN KEY (webhook_id) REFERENCES webhooks (id),
+    CONSTRAINT chk_targets_max_attempts CHECK (max_attempts >= 1)
 );
 
 CREATE INDEX idx_targets_deleted_at ON targets (deleted_at);