* feat: expose document_metadata in API and control plane
Add document_metadata (sourced from retain_params.metadata) to both
list and get document endpoints. Display it in the control plane
documents table and detail panel. Drop the unused metadata column
from the documents table (was always stored as empty {}).
* fix: code review fixes for document_metadata feature
- Remove unnecessary `import json as _json` (json already imported at module level)
- Simplify redundant truthiness checks in retain_params parsing
- Regenerate OpenAPI spec and client SDKs (Python, TypeScript, Go)
- Add tests for document_metadata in get_document and list_documents
* feat(ui): improve documents table and detail panel
- Relative timestamps with full date on hover
- Remove context column from table
- Metadata shown as k=v badges (blue, like tags)
- Size in bytes instead of chars
- Document IDs wrap instead of truncating
- Detail panel wider (560px)
- Retain params: context, event_date, metadata badges
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