Add stable persisted IDs for block-array items

[bobbwal]

Problem

Blocks in arrays such as pages.blocks have blockType and position, but no stable per-block ID. That means frontend
consumers can only target blocks by type or index, and index changes on reorder.

Repro

1. Create a page with multiple blocks.
2. Render them in Astro with DOM hooks.
3. Reorder blocks in the CMS.
4. Any index-based selector/hook now points at a different block.

Proposal

Add a persisted internal ID for each block item in discriminated block arrays.

Requirements:

• generated once when a block is created
• stored with the block data
• preserved on reorder/edit/save
• available in fetched content
• hidden from normal editorial UI if possible

Example

Current:

{ "blockType": "hero", "heading": "Welcome" }

Desired:

  { "blockId": "blk_abc123", "blockType": "hero", "heading": "Welcome" }

Acceptance criteria

• new block items receive a stable persisted ID
• reorder does not change the ID
• edits do not change the ID
• fetched content includes the ID
• existing content has a backfill/migration path