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>

README.md

@@ -24,7 +24,7 @@ send log and DNS checks in the panel, encrypted backups.
 - Web panel — domains, applications, deliveries, mail queue, system log, backup
 - Multi-domain relay — each SASL application is bound to one sending domain
 - DNS status checks (PTR, SPF, DKIM, DMARC) with in-panel re-check
-- Two-level rate limiting — IP backstop (Postfix) and per-domain/per-app limits
+- Two-level rate limiting — IP backstop (Postfix), per-domain ceilings, and trusted-IP app overrides
 - Full-server backup and single-domain export/import (optional password encryption)
 - Single Docker image; data in a `./data` bind mount