feat(dns): add custom DNS provider plugin system

- Add plugin interface with lifecycle hooks (Init/Cleanup)
- Implement thread-safe provider registry
- Add plugin loader with SHA-256 signature verification
- Migrate 10 built-in providers to registry pattern
- Add multi-credential support to plugin interface
- Create plugin management UI with enable/disable controls
- Add dynamic credential fields based on provider metadata
- Include PowerDNS example plugin
- Add comprehensive user & developer documentation
- Fix frontend test hang (33min → 1.5min, 22x faster)

Platform: Linux/macOS only (Go plugin limitation)
Security: Signature verification, directory permission checks

Backend coverage: 85.1%
Frontend coverage: 85.31%

Closes: DNS Challenge Future Features - Phase 5

frontend/src/api/dnsProviders.ts

@@ -58,10 +58,15 @@ export interface DNSProviderTypeInfo {
   fields: Array<{
     name: string
     label: string
-    type: 'text' | 'password'
+    type: 'text' | 'password' | 'textarea' | 'select'
     required: boolean
     default?: string
     hint?: string
+    placeholder?: string
+    options?: Array<{
+      value: string
+      label: string
+    }>
   }>
   documentation_url: string
 }