feat(endpoint-microsub): PR 1 - Core Microsub server with channels and timeline

[rmdes]

Summary

This PR adds the foundational Microsub endpoint with channel and timeline management.

What's included

Microsub API:

• GET/POST ?action=channels - list, create, update, delete, reorder channels
• GET/POST ?action=timeline - list items, mark read/unread, remove

Storage:

• MongoDB collections for channels and items
• Cursor-based pagination for timeline
• Per-user channel ordering and read state tracking

Features:

• Follows Microsub spec for channel and timeline actions
• Testable with existing Microsub clients (Monocle, Indigenous, etc.)
• Multi-user support via userId from session/token

How to test

1. Enable the plugin in your Indiekit config
2. Use a Microsub client like Monocle pointed at your /microsub endpoint
3. Or test via curl:

# List channels
curl -H "Authorization: Bearer YOUR_TOKEN" \
  "https://your-site.com/microsub?action=channels"

# Create a channel
curl -X POST -H "Authorization: Bearer YOUR_TOKEN" \
  -d "action=channels&name=Tech News" \
  "https://your-site.com/microsub"

# Get timeline (will be empty until feed fetching is added in PR 3)
curl -H "Authorization: Bearer YOUR_TOKEN" \
  "https://your-site.com/microsub?action=timeline&channel=CHANNEL_UID"

PR breakdown

This is PR 1 of 6 for the Microsub implementation:

• PR 1 (this): Core + Channels + Timeline API ← functional Microsub server
• PR 2: Feed discovery and subscription
• PR 3: Feed fetching and parsing
• PR 4: Reader UI
• PR 5: Compose and Micropub integration
• PR 6: Settings and filtering

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Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com