Fix welcome note 500 errors and improve serialization error handling

[TheZupZup]

Summary

This PR fixes a critical bug where seeded demo content (welcome note) on a fresh server would trigger 500 errors when a sync engine ran against the same data directory. It also improves error reporting for serialization failures throughout the sync pipeline.

Key Changes

• Demo data seeding improvements:

  • Extracted demo notebook/note creation into a new seed_demo_data() function in server.py
  • Mark seeded demo content with SyncStatus.SYNCED so sync engines don't attempt to re-push it to itself
  • Made seeding idempotent (only creates demo content if no user notebooks exist)
  • Updated ensure_default_notebook() to also mark the fallback notebook as SYNCED

• Serialization error handling:

  • Wrapped _serialize() methods in both NoteMetaResource and InkPageResource with try-catch blocks
  • Errors are now logged with context and converted to safe DAV errors instead of bare 500s
  • Added serialization error handling in _push_note() to catch failures during slug computation and payload serialization
  • Errors are now reported in sync reports with human-readable messages instead of tracebacks

• Test coverage:

  • Added live_server_with_demo fixture for testing against a server with seeded demo content
  • Added TestDemoSeed class to verify demo data is created with correct sync status and is idempotent
  • Added TestWelcomeNoteSync class with three scenarios:
    • Self-sync doesn't re-push demo notes
    • Fresh clients pull demo notes as SYNCED
    • Fresh clients can push welcome notes to empty servers
  • Added TestPushSerializationFailures to verify serialization errors surface useful error messages

Implementation Details

The root cause was that demo notes were created with default SyncStatus.LOCAL_ONLY, causing sync engines to treat them as new local content requiring push. When pushing to the same server, this could fail with unhelpful 500 errors. By marking demo content as SYNCED, it's treated as already-synchronized and skipped during push operations.

Serialization errors are now caught at multiple levels:

1. In WebDAV resource classes before content is sent to the client
2. In the sync client during path generation and payload preparation
3. All errors are logged with full context and reported in sync reports with actionable messages

https://claude.ai/code/session_01AydQTDSxkhVgZy2TVXuFDr