Add API endpoints and MCP server support for block indentation and hierarchical block nesting

Current Limitation

While users can manually indent blocks in the Capacities UI to create nested hierarchies and structured documents, the Capacities API and MCP server do not currently support setting or modifying block indentation levels.

Problem

Because block indentation in Capacities relies on an internal document hierarchy rather than on standard Markdown syntax, external integrations and AI assistants that use the MCP server cannot construct or reorganize nested outlines. As a result, text that is generated, edited, or refined is inserted strictly as flat text, requiring users to manually re-indent many sub-items, blocks, toggles, and nested lists.

Proposed Solution

Expose API capabilities to define or update a block's parent relationship or indentation depth (e.g., introducing a parentId or indentationLevel parameter when creating/updating blocks) and update the Capacities MCP server tools to support block nesting.

Key Benefits

• Native Outline Creation: Enables AI assistants and MCP tools to generate rich, hierarchically structured notes, nested lists, and structured sub-blocks automatically.
• Reduced Manual Formatting: Eliminates the need for users to frustratingly manually tab-indent blocks after AI generation or API block insertion.