feat: add file backend for writing emails to disk

[nklsw]

Description

• Add FileBackend that writes each outgoing email to its own file on
  disk - similar dev/inspection use case as ConsoleBackend, but persistent
• Support two output formats: .eml (RFC 5322, opens in Apple Mail /
  Thunderbird / VS Code preview) and .txt (identical to
  ConsoleBackend output)

Usage

# Default .eml format - opens in any mail client
  config = EmailConfig(
      backend=FileConfig(path="./tmp/emails"),
      from_email="noreply@example.com",
  )

  # Plain-text dump (matches ConsoleBackend output)
  config = EmailConfig(
      backend=FileConfig(path="./tmp/emails", format="text"),
      from_email="noreply@example.com",
  )