Email notifications + relief acknowledgement link

[guangshinhaha]

Context

src/lib/email.ts currently only logs OTPs to the console. Teachers receive zero notification when assigned to relief — they have to check the dashboard. Every competitor sends email at minimum.

Dive additionally pushes Telegram/WhatsApp; that's a separate P1 ticket. This issue covers email + the acknowledgement loop, which is the table-stakes baseline.

Scope

• Real email provider wired up (Postmark / Resend / string.sg notification service — pick whichever has MOE-friendly data residency).
• Send-on-assign: when a relief assignment is created or changed, notify the relief teacher with date, period, class, subject, room, and any lesson notes from the absent teacher.
• One-tap Acknowledge link in the email → marks assignment as acknowledged in DB → visible on admin dashboard.
• Resend / unacknowledged warning surfaced on dashboard.
• Notifier abstraction (`Notifier` interface) so Telegram/WhatsApp can slot in later without rewrites.

Acceptance criteria

• Assigning a teacher in src/app/api/relief-assignments triggers an email.
• Email contains a signed acknowledgement URL (no login required).
• `ReliefAssignment` has `acknowledgedAt` field; dashboard shows ✓ / pending.
• Admin can manually resend.
• OTP email path migrated to the same Notifier.

Refs

• docs/research/competitive-gap-analysis.md §4 items 2 & 4
• Eric's data-residency concern (§2 pain 6) — favour string.sg/MOE-hosted provider

[guangshinhaha]

Closed by commit 402a157 — email notifications + acknowledgement