Send a test of a broadcast or sequence email to yourself #34

There is no way to send a broadcast or sequence email to yourself before launching. The only "test" today checks the ESP connection (settings/esp/test.ts, MCP send_test_email). #26 wants to check that "a test send was made", but nothing can make one.

Proposal

• Add POST /sequences/:id/emails/:emailId/test with a list of recipient addresses (default: the current user, at most 5).
• Render exactly as a real send would (merge tags with sample or the sender's contact data, tracking, footer), with a [Test] subject prefix.
• Use a dummy contact id (for example all zeros) so the unsubscribe link renders validly but matches nobody. This is the approach in https://github.com/marclou/mailcheap.
• Record lastTestSentAt on the email so the Pre-send checklist before launching a broadcast #26 checklist can warn when no test was sent.
• Add a "Send test" button in the broadcast and sequence editor (shadcn dialog).
• Add the REST endpoint, an MCP tool and docs.