email: store emails.db per account under email_accounts/<account>/

Reorganizes email storage: the DB moves from DATA_PATH/<user>/emails.db to
DATA_PATH/<user>/email_accounts/<accountEmail>/emails.db, with a shared
email_accounts/attachment_cache/ (was Gmail/emails/attachments). openEmailDb now
takes (owner, account); a new openUserEmailDb(owner, userId) resolves the user's
configured account (first enabled) for read paths. Threads the account through
email.ts, accounts, resync, queue sync, channel handlers, and the email_db MCP
tool path. Drops the dead getUserEmailDir helper.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/migrate-emails-to-sqlite.ts

@@ -27,7 +27,8 @@ if (targetEmail) {
 function migrate(userEmail: string): void {
   console.log(`Migrating ${userEmail}...`);
   const emailDir = join(DATA_PATH, userEmail, 'Gmail', 'emails');
-  const db = openEmailDb(userEmail);
+  // Obsolete one-off migration (old .eml-file store → SQLite); kept only to compile.
+  const db = openEmailDb(userEmail, userEmail);
 
   let filenames: string[];
   try {