Add MailKite ESP backend

[bucabay]

What this adds

A new MailKite (mailkite.dev) backend, modeled closely on the existing Resend backend — MailKite's send API is JSON/shape-compatible with Resend's (POST JSON, Authorization: Bearer, {from,to,subject,html,text,…}), so the contribution follows the same structure you already accept and maintain.

MailKite is an inbound-first developer email platform: it sends transactional mail, and also receives email (parsed body + SPF/DKIM/DMARC verdict) delivered to a signed webhook. This PR implements the send side.

Anymail features supported

MailKite's API actually exposes a few things Resend's doesn't, so the backend supports more Anymail features than the Resend one:

Feature	MailKite	(Resend, for reference)
template_id + merge_global_data (server-side templates, {{merge_tags}})	✅	❌
track_opens / track_clicks (per-message override)	✅	❌
send_at (scheduledAt)	✅	✅
merge_metadata / merge_headers (batch send, POST /v1/send/batch)	✅	✅
metadata, tags (via X-Metadata / X-Tags headers)	✅	✅
attachments (content/filename/contentType)	✅	✅

Documented limitations (on the ESP page, raised as AnymailUnsupportedFeature)

• No outbound tracking webhooks — MailKite does not currently emit delivery/open/click/bounce events as webhooks, so AnymailTrackingEvent signals are not available. (Open/click tracking still works in MailKite's own dashboard, and per-message track_opens/track_clicks are supported.)
• No inbound via Anymail signals — MailKite does inbound (its headline feature), but delivers to the user's own webhook URL in its own payload format, so it isn't wired through AnymailInboundEvent.
• No inline attachments (Content-ID) — MailKite's send API has no cid field, so inline images raise rather than silently degrading to regular attachments.
• Single replyTo only — MailKite takes one reply-to string; multiple reply_to addresses raise.
• No envelope_sender.

How to configure

# settings.py
EMAIL_BACKEND = "anymail.backends.mailkite.EmailBackend"

ANYMAIL = {
    "MAILKITE_API_KEY": "mk_live_...",   # or MAILKITE_API_KEY / ANYMAIL_MAILKITE_API_KEY
}
# Optional: ANYMAIL = { ..., "MAILKITE_API_URL": "https://api.mailkite.dev/" }

No extra install — MailKite uses only Anymail's existing requests dependency (hence mailkite = [] in optional-dependencies, no [mailkite] extra).

How I tested it

• tests/test_mailkite_backend.py — 45 mocked tests covering standard Django email features and all the Anymail additions above, including the batch path (empty merge_data, merge_metadata, merge_headers) and a per-recipient rejection (MailKite reports batch failures per-recipient; verified they surface as "rejected").
• tests/test_mailkite_integration.py — live tests against the real API, guarded by ANYMAIL_TEST_MAILKITE_API_KEY / ANYMAIL_TEST_MAILKITE_DOMAIN (skip cleanly without them).
• Ran the full mocked suite locally (Django 6.0, Python 3.13): python runtests.py tests.test_mailkite_backend tests.test_mailkite_integration → 49 passed (4 skipped). The Resend backend tests still pass unchanged.
• pre-commit (black, isort, flake8, doc8, pygrep-hooks) is clean on every file in this PR.

I did not run the live integration tests (I don't want to charge a real MailKite account from a PR). If you'd like, I can run them against a MailKite test domain before merge, or you can run them in CI with credentials.

Files

• anymail/backends/mailkite.py
• tests/test_mailkite_backend.py, tests/test_mailkite_integration.py
• docs/esps/mailkite.rst (+ toctree, README, feature-matrix column, pyproject keywords/extra, changelog)

Maintenance

Happy to maintain this backend going forward — keeping it in step with MailKite's send API as it evolves, and adding outbound tracking webhook support (→ AnymailTrackingEvent) and Anymail inbound signal support if/when MailKite exposes the corresponding surfaces. I've read CONTRIBUTING and followed the existing ESP-backend pattern; happy to rework anything that doesn't match your conventions.

(Heads-up on the feature-matrix support-status row: I marked MailKite Unsupported per the footnote — Anymail doesn't have a MailKite account for live integration tests yet. Adjust to Limited/Full as you see fit once you have access.)

[bucabay]

Apologies — this duplicates #481, which is the maintained MailKite backend PR and the only one going forward (as promised there). This one was opened in error by our tooling; we've fixed the process so it can't happen again. Closing.