feat: Gmail batchModify (archive/read/labels)

[lexi-chief-of-staff]

Add batchModify method to GmailDriver using Gmail API users.messages.batchModify endpoint. Support bulk actions: archive (remove INBOX), mark read/unread, add/remove labels.

Spec:

• Input: array of message IDs + actions array (e.g. [['removeLabelIds' => ['INBOX']], ['addLabelIds' => ['archive']]])
• Rate-limit queue via Laravel jobs (max 1000 IDs/batch per API docs)
• Return: modified message IDs + any errors
• Tests: 100-msg mock, edge: invalid ID, rate-limit retry

Blocks Lexi Gmail triage tools. Refs # (Lexi issues TBD)