Update subscription methods to accept and use user access tokens: - Add userAccessToken parameter to SubscribeToCommunity() - Add userAccessToken parameter to UnsubscribeFromCommunity() - Add deleteRecordOnPDSAs() helper for user-scoped deletions - Use createRecordOnPDSAs() for subscription creation - Validate token presence before PDS operations This fixes the authorization issue where we were using instance credentials to write to user repositories, which the PDS correctly rejected with 401 errors. Now each user operation uses that user's own access token, ensuring proper atProto authorization semantics.