Feat: message sending-forwarding

[TatevikGr]

Summary by CodeRabbit

• New Features

  • Forward-to-friend workflow with per-recipient limits, admin notifications and multi-format delivery (HTML/Text/PDF).
  • Message precaching including optional user-specific remote content and faster per-subscriber sending.

• Improvements

  • Richer placeholder system, automatic URL absolutization, smarter image embedding and attachment handling.
  • Stronger campaign headers, delivery controls and admin/start observability.

• Bug Fixes

  • Mail-size enforcement, more reliable attachment copy/retries and clearer failure handling.

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