feat(inbound): custom receiving domains + catch-all addresses

[tuyakhov]

Summary

Follows up the base inbound support with the phase-2 capabilities now available
on the API, on both the sync and async clients:

• Custom receiving domains — receive mail on your own domain (e.g.
  inbound.acme.com) in addition to the shared receiving domain. Claiming a
  domain returns the DNS records to publish; it verifies once they're live.
• Catch-all addresses — a "*" local part receives every address no exact
  address claims.
• Address creation gains an explicit livemode and a domain_id to target a
  custom domain.

Stacked on the base inbound branch so the diff here is only the phase-2 delta.
Retargets to main automatically once the base branch merges.

Changes

• client.inbound.domains — list(), create(domain, ...), delete(id) on
  both Inbound and AsyncInbound. create() returns the DNS records to
  publish; a domain that already routes mail elsewhere raises a 409
SenderKitAPIError (existing_mx) so a caller can confirm before redirecting.
• inbound.addresses.create() gains domain_id and livemode; local_part="*"
  provisions a catch-all.
• New models: InboundDomain, InboundDnsRecord.
• README gains a custom-domains / catch-all example.

Checklist

• ruff check . passes
• ruff format . applied
• mypy src passes
• pytest passes (new behavior is covered by tests)
• PR title / commits follow Conventional Commits
• Docs / docstrings updated

This mirrors the matching change in the TypeScript and PHP SDKs so all three
stay in sync.

---

Generated by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.43750% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/senderkit/resources/inbound.py	97.56%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[tuyakhov]

CI status: the red checks are ruff format --check (the Lint & type-check job) and the aggregate CI gate that depends on it. The reformat it wants is in pre-existing docstring/example code (the integrations/ framework examples) — files untouched by this change.

It's pre-existing on the base branch: this PR is stacked on the inbound base branch, whose CI fails identically, so it needs to clear there. This phase-2 diff itself passes ruff check, mypy src, and pytest locally — so I'm not reformatting unrelated files in this feature PR.

---

Generated by Claude Code

[tuyakhov]

Update — root-caused and fixed. The red ruff format gate was ruff 0.16 (which CI's floating ruff>=0.5 now resolves to) formatting Python code blocks inside Markdown by default, so README.md got reformatted. Local runs and the pinned pre-commit hook (0.15.17) don't do this.

Fix: pinned ruff==0.15.17 in the dev extra to match the pre-commit hook (verified: clean under 0.15.17, reformats under 0.16). Pushed here so this PR's lint gate goes green, and opened the same one-line fix against the base branch in #15 so it's fixed at the source too — the pins are identical, so they merge cleanly.

---

Generated by Claude Code