BREAKING CHANGE(rust-bridge): make Rust the primary security backend, remove all TS fallbacks

Phase 3 of the Rust migration: the Rust security bridge is now mandatory
and all TypeScript security fallback implementations have been removed.

- UnifiedEmailServer.start() throws if Rust bridge fails to start
- SpfVerifier gutted to thin wrapper (parseSpfRecord stays in TS)
- DKIMVerifier gutted to thin wrapper delegating to bridge.verifyDkim()
- IPReputationChecker delegates to bridge.checkIpReputation(), keeps LRU cache
- DmarcVerifier keeps alignment logic (works with pre-computed results)
- DKIM signing via bridge.signDkim() in all 4 locations
- Removed mailauth and ip packages from plugins.ts (~1,200 lines deleted)

ts/plugins.ts

@@ -84,16 +84,10 @@ export {
 }
 
 // third party
-import * as mailauth from 'mailauth';
-import { dkimSign } from 'mailauth/lib/dkim/sign.js';
 import mailparser from 'mailparser';
 import * as uuid from 'uuid';
-import * as ip from 'ip';
 
 export {
-  mailauth,
-  dkimSign,
   mailparser,
   uuid,
-  ip,
 }