trace: normalize OTLP and AgentV artifacts for Dashboard

[christso]

Convert AgentV trace sidecars and OTLP JSON files into the Dashboard read model so the UI can render Phoenix-style trace trees without parsing raw OTLP directly.

[Necmttn]

For this normalization pass, I would keep the raw and dashboard layers separate.

Raw OTLP/AgentV events should stay immutable, keyed by their source event id plus trace/span ids where present. The dashboard tree can then be a materialized read model over those rows, not the only persisted form of the trace.

That buys two things:

• idempotent re-projection when the AgentV or OTLP mapping changes
• late correlation to sessions/artifacts without pretending telemetry and transcripts arrive in order

The useful metadata to persist on the projection is probably source, source_event_id, trace_id, span_id, parent_id, projection_version, and raw_parser_version. If the UI later needs a different tree shape, you can rebuild it without losing provenance.

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