Complete security hardening: zero-migration foundation, edge headers, rate-limit atomics, body limits

- Make @/lib/safe-action re-export from foundation layer so all 13+
  existing server actions instantly get request tracing, rate limiting,
  and structured error handling without any code changes
- Add HSTS, CSP, X-Frame-Options, X-Content-Type-Options to edge proxy
  (src/proxy.ts) — ran at Cloudflare/Vercel edge for all non-asset routes
- Fix rate-limit.ts race condition: compute newCount before assignment
  to shrink the read-modify-write window; add memory-key prefix to
  avoid collisions with Redis keys
- Add request body size limit (10 MB default) to api-handler.ts with
  per-route override via maxBodyBytes option
- Remove unused imports and clean up backward-compat types

src/lib/foundation/types.ts

@@ -31,7 +31,7 @@ export interface AdminActionContext extends ActionContext {
 
 export interface ActionSuccess<T = Record<string, unknown>> {
   ok: true;
-  data: T;
+  data?: T;
 }
 export interface ActionFailure {
   ok: false;