docs: send Supabase auth emails through SESKit

[Otitodev]

What and why

Supabase Auth sends its own emails from a shared mailer meant for development. This adds a guide and a small Supabase Edge Function that hand those emails to SESKit instead, so they go out from the user's domain and appear in the dashboard, events and suppression list.

• The function (docs/guides/supabase/send-email.ts) checks Supabase's Standard Webhooks signature, builds the confirmation link, and calls POST /v1/emails with an Idempotency-Key. Templates cover signup, invite, magic link, recovery, email change (two emails when secure), email OTP and reauthentication.
• The guide (docs/guides/supabase-auth-emails.md) covers setup, what is sent, and how failures behave.

What to review hardest:

• Failure behaviour. Supabase allows the hook 5 seconds and retries a 503. SESKit being slow or down gives 503, and the idempotency key makes the retry safe. A refusal gives 400.
• email_change. Supabase names its token pairs backwards. The code follows their documented mapping, and a test covers the case where the unused pair arrives as empty strings.
• Nothing sensitive in logs (no address, token or link), and every value in the HTML is escaped.

Not done, and worth knowing:

• Not run against a live Supabase project. The guide says so in a caution box, to be removed after a real signup, reset and magic link.
• The function is TypeScript, because Supabase Edge Functions run on Deno. AGENTS.md says no Node.js; this runs on Supabase and needs no toolchain here, but it is a judgement call.
• No tests in the repo. CI cannot run TypeScript. I ran 18 tests locally with Node's built-in runner, outside the repo. The code has not been type-checked.
• Unverified: whether enabling the hook lifts Supabase's own email rate limits; the docs do not say.
• May conflict slightly with feat: turn receiving on, and read it in the dashboard (inbound Phase D) #63 in mkdocs.yml.

Checks

• Docs: updated, and uv run --group docs mkdocs build --strict is clean

🤖 Generated with Claude Code