Sentry: verify @comapeo/core OTel span forwarding (Phase 4)

[gmaclennan]

Part of #74. Blocked on comapeo-core#1051 landing. Verification only — most useful once #75's debug toggle exists, because core's spans need a per-RPC parent span to attach to (§11.3).

Spec: docs/sentry-integration-plan.md Phase 4.

@comapeo/core is gaining OpenTelemetry instrumentation — spans describing its internal operations (sync, indexing, hypercore). Once that lands, Sentry should pick those spans up automatically and nest them under our RPC traces, giving deep visibility inside core operations.

Deliverables

• Bump @comapeo/core in backend/package.json once PR #1051 ships
• With debug on, verify core's spans appear in Sentry nested under the RPC transaction
• Document any tracing-config overrides needed
• Confirm the full chain: JS client span → backend RPC transaction → core operation spans