fix: address code review findings for DNS management

- CRITICAL: Change DNS zones endpoint from GET to POST to avoid
  leaking API token in URL query parameters
- HIGH: Add sync.RWMutex to protect dnsProvider field in Server,
  Deployer, and proxy Manager against concurrent read/write races
- HIGH: Capture old DNS provider reference synchronously before
  launching background cleanup goroutine
- HIGH: Use getDNS()/getDNSProviderLocked() accessors instead of
  direct field reads in all DNS operations

web/.svelte-kit/generated/client-optimized/nodes/6.js

@@ -0,0 +1,3 @@
+import * as universal from "../../../../src/routes/events/+page.ts";
+export { universal };
+export { default as component } from "../../../../src/routes/events/+page.svelte";