Authenticated, scoped sync: signed requests and per-path subscriptions

- RequestAuth: clients sign `${timestamp}:${path}:${body}` (as in
  sync-server-example); headers carry public key, timestamp, signature and
  algorithm. Pluggable signers/verifiers; Ed25519 via WebCrypto built in.
  Timestamp window and replay protection.
- KVStorageHttpServer requires auth ('none' must be explicit). With auth, every
  key, scan range and subscription path must be under the caller's public key.
- POST /events with { paths }: subscribers only receive (and on resume only
  replay) changes under their authorized paths.
- KVStorageHttp: signer option; subscribe(path)/unsubscribe(path)/close()
  replace watch(); 401/403 reject instead of retrying.
- KVStorageComposite: prefix option scopes sync, cache and subscription.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

src/utils/errors.ts

@@ -25,3 +25,14 @@ export class StorageUnavailableError extends Error {
     this.name = 'StorageUnavailableError'
   }
 }
+
+/** A request was not authenticated (401) or not allowed for this caller (403). */
+export class AuthError extends Error {
+  constructor(
+    readonly status: 401 | 403,
+    message: string,
+  ) {
+    super(message)
+    this.name = 'AuthError'
+  }
+}