add blocks api (block_types, block_schemas, block_documents)
implements full blocks support with:
- block_types: create, filter, get by slug, update
- block_schemas: create, filter, get by checksum/id (server-side sha256 checksum)
- block_documents: create, filter, get, update, delete
uses std.json.Stringify streaming pattern for json serialization.
health endpoint now returns "ok" (text/plain) to match python server.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>