Support Saved Notes
Adds full support for Substack's saved notes alongside posts — sync, search, list, save/unsave, and full-content fetch, backed by a dedicated notes table and FTS5 index. All notes operations use direct authenticated API calls, no browser required. New CLI commands (list-notes, search-notes, note-authors, save-note, unsave-note, get-note) plus matching MCP tools and resource. Also fixes a few latent bugs in the posts code: a crash on URL-changing upserts, filters silently dropped by search's fallback path, and inaccurate sync timestamps.