feat(vNext.4): implement trusted-subject security chain for portal user key self-service

- Add portal_auth.go: Portal user session auth with HMAC-signed cookies
- Add /api/portal/session/{login,logout,state} endpoints
- Update nginx config template: cookie-to-header trusted proxy pattern
- Update frontend: sync CRM session on login/logout
- Add TRUSTED_SUBJECT_DEPLOY_GUIDE.md with remote43 deployment steps
- Update EXECUTION_BOARD.md: mark trusted-subject blocking issue as resolved

This implements the secure chain:
  Browser → Portal → nginx (cookie→header) → CRM (verify proxy secret)

Required remote43 actions:
1. Generate 64-char hex secret
2. Update .env.crm with TRUSTED_* config
3. Update nginx with cookie map and header injection
4. Restart services

Fixes EXECUTION_BOARD.md 2026-06-08 blocking issue

.env.example

@@ -8,3 +8,9 @@ SUB2API_CRM_ROUTE_RUNTIME_BACKEND=memory
 SUB2API_CRM_REDIS_ADDR=
 SUB2API_CRM_REDIS_PASSWORD=
 SUB2API_CRM_REDIS_DB=0
+
+# Trusted Subject Proxy Configuration (for portal user key self-service)
+# These must be synchronized with nginx config
+SUB2API_CRM_TRUSTED_SUBJECT_HEADER=X-CRM-Authenticated-Subject
+SUB2API_CRM_TRUSTED_PROXY_SECRET_HEADER=X-CRM-Trusted-Proxy
+SUB2API_CRM_TRUSTED_PROXY_SECRET=change-me-64-char-hex-secret-for-production