add syndication text field with 240-char limit

Created by: jedmund

Summary

• Added a syndicationText field to posts so you can write a custom message for Bluesky/Mastodon instead of relying on auto-generated text
• Rewrote syndication formatting to use a single 240-character limit across all platforms and content types
• Added excerpt column to the Post model for essay summaries

What changed

Database — new excerpt and syndicationText columns on the Post model

Admin UI — syndication message textarea in post metadata (both full form and popover), with char count and 240 max

Syndication logic — simplified the formatting priority:

1. Custom syndicationText if set
2. excerpt (for essays/posts that have one)
3. Auto-generated from content (truncated to 240 chars)

Removed the old per-platform, per-content-type excerpt limits in favor of one consistent 240-char cap with a shared truncateText helper.

API routes — create/update/patch endpoints now accept and persist both new fields

Test plan

• Create a post with a custom syndication message → publishes with that text on Bluesky/Mastodon
• Create a post without syndication message → falls back to excerpt or content
• Essay with excerpt → syndication uses title + excerpt, truncated to 240 chars
• Char counter in the textarea shows correct count and enforces 240 limit
• Edit an existing post → syndicationText loads and saves correctly