feat: registry-driven DNS provider type discovery

Phase 1 of Custom DNS Provider Plugin Support: the /api/v1/dns-providers/types
endpoint now returns types dynamically from the dnsprovider.Global() registry
instead of a hardcoded list.

Backend handler queries registry for all provider types, metadata, and fields
Response includes is_built_in flag to distinguish plugins from built-ins
Frontend types updated with DNSProviderField interface and new response shape
Fixed flaky WAF exclusion test (isolated file-based SQLite DB)
Updated operator docs for registry-driven discovery and plugin installation
Refs: #461

frontend/src/hooks/useDNSProviders.ts

@@ -11,6 +11,7 @@ import {
   type DNSProvider,
   type DNSProviderRequest,
   type DNSProviderTypeInfo,
+  type DNSProviderField,
   type DNSTestResult,
 } from '../api/dnsProviders'
 
@@ -111,5 +112,6 @@ export type {
   DNSProvider,
   DNSProviderRequest,
   DNSProviderTypeInfo,
+  DNSProviderField,
   DNSTestResult,
 }