feat(frontend): propagate trace context via OpenAPI interceptor

[ncolesummers]

Summary

• add frontend/src/telemetry-interceptor.ts to inject W3C trace context (traceparent/tracestate) into every generated OpenAPI client request via OpenAPI.interceptors.request
• register the interceptor at startup in frontend/src/main.tsx immediately after telemetry initialization
• update observability docs to explain the explicit educational propagation pattern and troubleshooting step for interceptor registration

Why

Issue #6 requires frontend-to-backend distributed trace continuity without modifying generated client files under frontend/src/client/.

Validation

• bun run --cwd frontend build passes
• Full stack runtime verification after container restart:
  • captured browser request header: traceparent: 00-9d901eba971201dd7b4e5ed1a8f87529-22ed13efe1b40b13-01
  • Jaeger trace 9d901eba971201dd7b4e5ed1a8f87529 includes both services (react-frontend, fastapi-backend)
  • backend server span GET /api/v1/users/me has parent span ID 22ed13efe1b40b13 (matching the frontend client span ID from traceparent)

Closes #6

[Copilot]

Pull request overview

Adds an explicit OpenTelemetry trace-context propagation hook to the frontend’s generated OpenAPI client so backend requests carry W3C traceparent/tracestate without modifying generated client sources.

Changes:

• Introduce frontend/src/telemetry-interceptor.ts to inject W3C trace context into OpenAPI client requests via OpenAPI.interceptors.request.
• Register the interceptor during app startup in frontend/src/main.tsx after telemetry initialization.
• Update docs/observability.md to describe the propagation approach and add a troubleshooting check.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
frontend/src/telemetry-interceptor.ts	New interceptor + one-time registration helper to inject traceparent/tracestate into request headers.
frontend/src/main.tsx	Calls registerTraceContextInterceptor() after initTelemetry().
docs/observability.md	Updates trace propagation explanation and adds a troubleshooting step for interceptor registration.

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