feat(inbound): MailgunInboundParser + DI registration

[mpge]

Summary

Adds Mailgun as the second supported inbound provider alongside Postmark (from #24). Mailgun POSTs multipart/form-data with snake-case field names (sender / recipient / body-plain / body-html / Message-Id / In-Reply-To / References / attachments). The controller's signature already accepts any dictionary-shaped payload, so no controller changes needed — register the new parser and route via ?adapter=mailgun.

Notes

• Mailgun's from is typically "Name <email>" — we extract the display name portion separately and fall back to the sender field for the email.
• Mailgun hosts attachment content behind a URL (large attachments). We carry the URL through in DownloadUrl; a follow-up worker can fetch + persist out-of-band.
• Malformed attachments JSON degrades gracefully (empty list, no exception propagation).

Dependencies

• Stacked on feat(inbound): PostmarkInboundParser + InboundEmailController + DI wiring #24 (feat/inbound-email-controller). Merge order: feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #21 → feat(email): wire MessageIdUtil into EmailTemplates + signed Reply-To #22 → feat(inbound): scaffold InboundMessage + IInboundEmailParser + InboundEmailRouter #23 → feat(inbound): PostmarkInboundParser + InboundEmailController + DI wiring #24 → this PR.

Test plan

• 7 xUnit tests cover core field extraction, threading headers, provider-hosted attachment parsing, malformed attachments JSON, sender→from fallback, bare-email (no display name) handling, and the adapter name contract
• CI green (won't trigger against stacked base until rebased)