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/web/handlers/handlers.go

@@ -24,6 +24,10 @@ type Config struct {
 	TLSCertFile       string
 	OpenDKIMSocket    string
 	JournalSocket     string
+	// Level-1 Postfix anvil backstop (env RATE_LIMIT_*), shown in the panel
+	// and used to cap domain/app level-2 ceilings (guide § Rate limiting).
+	RateLimitMessagesPerIP int
+	RateLimitWindowSeconds int
 }
 
 // Handlers holds dependencies for authenticated panel routes.