feat: Gmail batchDelete

[lexi-chief-of-staff]

Add batchDelete method to GmailDriver using Gmail API users.messages.batchDelete endpoint.

Spec:

• Input: array of message IDs (max 1000/batch)
• Permanent delete (no trash)—confirm via param permanent=true
• Rate-limit queue via Laravel jobs
• Return: deleted IDs + errors
• Tests: mock batch, single ID, oversized batch split

Enables Lexi bulk cleanup (spam/tests). Careful: no undo.