feat: submitBulk + getBulkStatus (async /email/bulk)

[jaballer]

Context

v2.0 covers Postmark's synchronous batch endpoints; the asynchronous bulk API (/email/bulk, submit→ID→poll, no count cap, approval-gated) is uncovered.

Proposal

Two tools: submitBulk (returns request ID) + getBulkStatus (poll by ID). The Postmark Node SDK (v4) doesn't expose /email/bulk → either drop to raw HTTP or contribute upstream. Document the multi-turn "I started the job, check back" UX.

Acceptance criteria

• Tools wrap submit + poll
• Multi-turn flow documented; approval-gating noted

SDK gap is the main cost. Source: roadmap (MCP/CLI analysis).