Add API endpoints and MCP tools for in-place block editing

Current Limitation

The Capacities API currently supports appending new blocks and updating frontmatter properties (such as title and tags), but lacks endpoints to modify or replace existing body blocks in place.

Problem

When using the Capacities MCP server, for example, to restructure and edit existing notes, AI assistants cannot make inline edits. Instead, they can only append revised text to the end of a page or create a duplicate page with edits, leaving users to clean up old, stale content manually.

Proposed Solution

Expose API endpoints that support updating or replacing existing body blocks by block ID (e.g., PATCH /blocks/{id} or/and a bulk block update endpoint).

Key Benefits

• Seamless AI Co-authoring: Allows MCP servers and AI assistants to perform inline rewrites, grammar corrections, and structural formatting inside existing notes.
• Better Integration Capabilities: Enables external sync tools to update specific block contents without appending duplicate entries.
• Clutter-Free Notes: Eliminates the need to append revision blocks or duplicate pages to reflect updates.