docs: add serve mode documentation (REST API + X-API-Key)

[Copilot]

gomailtest serve had no user-facing documentation — no startup instructions, no endpoint reference, no request/response examples, and no mention of the required X-API-Key header.

Changes

• docs/protocols/serve.md — new doc covering:

  • Starting the server (--api-key / SERVE_API_KEY, --port, --listen)
  • X-API-Key auth: how to pass it, what a missing/invalid key returns
  • Endpoint table (GET /health, GET /, POST /smtp/sendmail, POST /msgraph/sendmail, POST /ews/sendmail)
  • Request body schemas for SMTP and MS Graph send endpoints
  • SMTP* and MSGRAPH* env vars consumed at startup
  • HTTP status code reference
  • curl and PowerShell examples

• README.md — adds link to docs/protocols/serve.md in the Protocol Docs section

Example

export SERVE_API_KEY="mysecretkey"
export SMTPHOST="smtp.example.com" SMTPPORT="587"
export SMTPUSERNAME="user@example.com" SMTPPASSWORD="pass" SMTPFROM="sender@example.com"

gomailtest serve

curl -X POST http://localhost:8080/smtp/sendmail \
  -H "X-API-Key: mysecretkey" \
  -H "Content-Type: application/json" \
  -d '{"to":["recipient@example.com"],"subject":"Test","body":"Hello"}'
# → {"status":"ok"}