Feat/email building with attachments

[TatevikGr]

Summary by CodeRabbit

• New Features

  • PDF generation for messages, per-subscriber remote-content fetching, tracking-pixel user tracking, and richer attachment handling with downloadable copies.

• Improvements

  • Unified email builder flow with consistent composition and multi-format output (HTML/Text/PDF); expanded, context-aware placeholder personalization (many URL/list resolvers); improved remote-content precaching and output formatting; new configurable parameters and translations.

• Tests

  • Extensive unit tests added/updated to cover builders, placeholder resolvers, attachment handling, and new utilities.

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