feat: DNS provider abstraction — Technitium, Cloudflare, RFC 2136, Manual

- dns-providers/: adapter base class + registry with auto-discovery
- technitium.js: wraps existing Technitium API calls into adapter interface
- cloudflare.js: Cloudflare API v4 adapter (zones, records, credentials)
- rfc2136.js: RFC 2136 dynamic DNS via nsupdate (BIND, PowerDNS, etc.)
- manual.js: no-op adapter for external DNS management with instructions
- provider-dns.js: provider-aware DNS context, resolves active adapter from config
- Universal helper methods: universalCreateRecord/Delete/ResolveRecord
- All 7 route files updated to use universal methods instead of raw dns.call()
- Setup wizard: provider dropdown (Technitium, Cloudflare, RFC 2136, Manual)
- DNS template selector: added Cloudflare and External/Manual options
- Config schema: validates dns.provider field
- Capability gating on Technitium-specific endpoints (logs, restart, update)
- Backward compatible: no provider set = auto-detect (technitium if dns.ip exists)

dashcaddy-api/config-schema.js

@@ -59,6 +59,15 @@ function validateConfig(config) {
           errors.push('dns.servers must be an object');
         }
       }
+      // DNS provider validation
+      if (config.dns.provider !== undefined) {
+        const validProviders = ['technitium', 'cloudflare', 'rfc2136', 'manual'];
+        if (typeof config.dns.provider !== 'string') {
+          errors.push('dns.provider must be a string');
+        } else if (!validProviders.includes(config.dns.provider)) {
+          warnings.push(`dns.provider "${config.dns.provider}" is not one of: ${validProviders.join(', ')}. It may still work if a custom adapter is installed.`);
+        }
+      }
     }
   }