feat(metadata): extract recording index metadata from MCAP

[05F3759DF]

Context

Axon will make MCAP the source of truth for recording metadata and make JSON sidecars optional. Keystone should ingest MCAP files directly and reconstruct the existing sidecar-equivalent metadata from MCAP metadata records.

Scope

• Parse Axon MCAP metadata records for task, device, recording, and topic summary fields.
• Reconstruct the sidecar-equivalent metadata object from MCAP without requiring a .json file.
• Store extracted fields in Keystone's recording or episode index model.
• Preserve compatibility with existing uploads that still include JSON sidecars.
• Surface extraction failures with actionable diagnostics.

Acceptance Criteria

• Keystone can ingest an MCAP-only upload and populate the same metadata fields previously provided by sidecar JSON, excluding checksum.
• Keystone handles task, device, recording, and topics summary metadata consistently.
• If both MCAP metadata and JSON sidecar are present, Keystone can compare or prefer a documented source without creating inconsistent records.
• Extraction failures are recorded and visible for operations diagnostics.
• Tests cover MCAP-only ingestion, MCAP+JSON compatibility, malformed metadata, missing optional fields, and topic summary parsing.

Related

• feat(metadata): ensure MCAP contains sidecar-equivalent metadata axon#95
• feat(metadata): make recorder sidecar JSON generation optional axon#96
• feat(transfer): support MCAP-only scanning and upload readiness axon#97