feat(admin): session probe, self-hosted signup and the relink cap

Adds GET /auth/me so the admin site's route guards can know who is signed
in, POST /auth/signup for self-hosted customers, and max_relinks on the
account payload so the UI never hardcodes a rule the backend enforces.

Signup follows sitesvc's proven shape: honeypot answered as success, a
generic 201 when the address already exists, and nothing usable until the
emailed link is opened.

Also fixes a lockout found while verifying it. When the verification email
failed, the account was rolled back but the customer_users row survived --
an orphan that can never be signed in to and that holds the unique index on
email, so every later signup with that address got a cheerful 201 and the
customer was locked out of their own address with no visible error.
CreateCustomerUser now undoes its own insert.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/internal/config/config.go

@@ -61,10 +61,10 @@ func Load() (Config, error) {
 		// and must leave the username empty.
 		RedisUsername: os.Getenv("REDIS_USERNAME"),
 		RedisPassword: os.Getenv("REDIS_PASSWORD"),
-		SigningKey:      os.Getenv("LICENSE_SIGNING_KEY"),
-		PublicURL:       strings.TrimSuffix(os.Getenv("PUBLIC_URL"), "/"),
-		TrustProxy:      strings.EqualFold(os.Getenv("TRUST_PROXY"), "true"),
-		Addr:            ":" + envOr("PORT", "8083"),
+		SigningKey:    os.Getenv("LICENSE_SIGNING_KEY"),
+		PublicURL:     strings.TrimSuffix(os.Getenv("PUBLIC_URL"), "/"),
+		TrustProxy:    strings.EqualFold(os.Getenv("TRUST_PROXY"), "true"),
+		Addr:          ":" + envOr("PORT", "8083"),
 
 		SMTPHost:     os.Getenv("SMTP_HOST"),
 		SMTPPort:     envOr("SMTP_PORT", "587"),