Feature: Support scheduled thread publishing

[hellno]

Summary

Add support for scheduling threads (multi-post compositions) to be published at a future time.

Background

Thread scheduling was deferred from #668 (draft store thread support) because of complexity:

1. Pre-encoding limitation: Current scheduled casts are pre-encoded at draft time using preEncodeCastMessage(). This works for single posts but not for threads because:

   • Posts 2-N in a thread need parentCastId pointing to post 1's hash
   • Post 1's hash doesn't exist until it's published
   • Therefore, posts 2-N cannot be pre-encoded at scheduling time

2. Dependency chain: Thread posts must be published sequentially, with each subsequent post referencing the first post's hash.

Potential Solutions

Option A: Runtime Encoding (Simplest)

• Schedule the thread metadata only (not pre-encoded bytes)
• Edge function encodes all posts at publish time
• Requires edge function to have access to account's private key (already does for fallback)
• Con: Slightly less reliable than pre-encoded (but fallback already exists)

Option B: Staged Publishing

• Pre-encode and publish post 1 at scheduled time
• Edge function immediately publishes posts 2-N after post 1 succeeds
• Pro: Post 1 is pre-encoded for reliability
• Con: More complex edge function logic

Option C: Chained Scheduling

• Schedule only post 1 initially
• After post 1 publishes, automatically schedule posts 2-N
• Pro: Each post is scheduled independently
• Con: User experience unclear, timing may drift

Acceptance Criteria

• User can schedule a thread for future publication
• All posts in thread are published sequentially at scheduled time
• Partial failures are handled gracefully (same as immediate publishing)
• Thread scheduling state is persisted to Supabase

Dependencies

• Requires Refactor: Extend draft store to support thread composition #668 (draft store thread support) to be completed first

Related Issues

• Feature: Thread composer with drag-and-drop reordering #666 (Thread composer UI)
• Refactor: Extend draft store to support thread composition #668 (Draft store thread support)