Add embeddable signup forms with double opt-in support

[sagnik11]

TL;DR
Adds complete embeddable signup forms feature including a form builder UI, hosted form pages at /f/:id, a drop-in JS embed snippet, and optional double opt-in flow with confirmation emails and token-based verification.

What changed?

• Documentation updates

  • Added signup forms to CHANGELOG.md with brief feature description
  • Added comprehensive "Signup Forms" section to README.md covering form creation, sharing (hosted pages and JS embeds), double opt-in flow, and suppression/abuse protection
  • Marked "Embeddable signup forms" as completed in ROADMAP.md

• UI changes

  • Added "Forms" navigation item to dashboard sidebar with form icon

• Database schema

  • Added confirmationToken field to contacts table to support double opt-in verification
  • Updated contacts status field documentation to include new pending status (in addition to existing active, bounced, unsubscribed)
  • Added database migration (0002_quiet_nighthawk) to support new schema changes

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