move labeler cache to mmkv, make moderation config sync

The per-account labeler cache was the only async dependency in
configureModerationForAccount. MMKV-backed sync reads let session
setup apply labelers in the same tick. No AsyncStorage backfill: the
cache is rewritten on every preferences fetch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/storage/schema.ts

@@ -108,4 +108,15 @@ export type Account = {
    * account after a switch, until that account's preferences loaded.
    */
   isBetaUser?: boolean
+
+  /**
+   * The account's subscribed labeler DIDs, cached from preferences so the
+   * `atproto-accept-labelers` header can be configured synchronously at
+   * session start, before preferences load. Eventually consistent: rewritten
+   * on every preferences fetch (see `saveLabelers` in
+   * `#/state/session/moderation`). Until the first fetch lands there is
+   * simply no cache entry and initial requests go out without per-account
+   * labeler headers.
+   */
+  labelers?: string[]
 }