Implement inbound email ingestion

[grimicorn]

Priority: P2

What needs to happen

The UI advertises clip-xxxx@in.markpost.io email-in addresses (app/pages/sources.vue:106, app/components/AddSourceModal.vue), but there is no inbound email handling.

• Pick an inbound-email provider that posts parsed messages to a webhook (e.g. Postmark inbound, SendGrid Inbound Parse, or Mailgun routes). State the choice in the PR.
• Add server/api/email/inbound.post.ts to receive the provider's parsed-email webhook payload.
• Exclude this path from Clerk auth in server/middleware/auth.ts (same as the hooks route); authenticate the provider via a shared secret or signature.
• Map the recipient address (clip-xxxx) to a sources row → userId.
• Strip tracking pixels and convert subject + body (HTML or text) to a markdown record via Build the payload-to-Markdown conversion pipeline #51; insert into records.
• Update the source's lastHitAt / recordCount and emit an activity event (Persist activity events and expose an events API #59).
• Add the provider credentials to .env.example, .env.e2e.example, and any other sample env files, with comments explaining where to obtain them, and document the integration in README.md.
• Add unit tests under tests/.

Dependencies

• Expand the records schema for inbound source data #46 (expand-records-schema)
• Build the sources table and CRUD API #47 (sources-backend-api)
• Build the payload-to-Markdown conversion pipeline #51 (markdown-conversion-pipeline)