rate limit: domain ceiling for all IPs, trusted app override

Invert level-2 semantics so domain limits apply to every client IP and
application limits with trusted IPs raise the ceiling above the domain
(still capped by level 1). Panel shows L1, validates maxima, and documents
the model on Settings.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/domain/service.go

@@ -145,13 +145,13 @@ func (s *Service) RateLimit(domainID int64) (store.RateLimit, bool, error) {
 }
 
 // SaveRateLimit stores the domain-level rate limit. The caller has validated the
-// IPs and numbers (security.md); the milter reads the row live, so no reload is
-// needed.
+// numbers (security.md); the milter reads the row live, so no reload is needed.
+// Domain limits do not use an IP allowlist.
 func (s *Service) SaveRateLimit(domainID int64, ips []string, maxMessages, windowSeconds int) error {
 	return s.store.SetRateLimit(store.RateLimit{
 		Scope:         store.RateLimitScopeDomain,
 		RefID:         domainID,
-		AllowedIPs:    ips,
+		AllowedIPs:    ips, // unused for domain enforcement; kept empty by the panel
 		MaxMessages:   maxMessages,
 		WindowSeconds: windowSeconds,
 	})