Add Feedback-ID, List-Id and Precedence: bulk headers to broadcast and sequence sends #48

Background

Broadcast and sequence sends call sendMail with no extra headers (automation/process-ongoing-sequence.ts:321). There's no Feedback-ID, List-Id or Precedence: bulk.

Why it matters:

• Gmail's bulk-sender guidelines recommend Feedback-ID for the Postmaster Tools feedback loop.
• List-Id (RFC 2919) lets mailbox providers and readers identify a list.
• Precedence: bulk stops auto-replies to campaigns.
• Embedding platforms need a per-tenant Feedback-ID to see which tenant is drawing complaints.

Proposed fix

• Set Feedback-ID: <campaignId>:<teamId>:<type>:<senderId>, List-Id: <name> <teamId.list-domain> and Precedence: bulk on every broadcast and sequence email.
• Leave transactional mail alone unless the caller sets these headers.

Related: #23 (same code path).

Acceptance criteria

• Every broadcast and sequence send carries all three headers.
• Transactional sends don't get them by default.
• Unit tests assert the headers.