feat(ping): propagate trace context in cron monitor pings

[notheotherben]

Summary

Adds W3C trace headers (e.g. traceparent) to the HTTP requests sent to the configured cron monitor URLs (start / success / failure). This lets the monitoring service's telemetry be correlated with the originating backup run, enabling cross-service tracing when things fail — exactly the goal described in the request.

What changed

• src/telemetry/trace_propagation.rs (new): a small, reusable helper built on the OpenTelemetry text map propagator that the OpenTelemetry battery already installs globally.
  • A private HeaderInjector that writes propagated fields into a reqwest::header::HeaderMap, skipping anything that isn't a valid HTTP header.
  • A TracePropagationExt extension trait adding .with_trace_context() to reqwest::RequestBuilder, which injects the current span's trace context into the outgoing request.
• src/ping.rs: the cron-monitor GET requests now call .with_trace_context().
• src/telemetry/mod.rs: re-exports the new module.

When telemetry export is disabled (no valid span context), no headers are added, so requests are left untouched.

Notes

• The injector is intentionally generic on reqwest::RequestBuilder so other outbound HTTP clients (GitHub/Forgejo) can adopt the same propagation later if desired.

Testing

• cargo build ✅
• cargo test --features pure_tests ✅ (109 passed; network-gated tests ignored as usual)
• New unit tests verify a valid context produces the correct traceparent header and that an empty context produces none.
• cargo fmt --check and cargo clippy --all-targets ✅

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.18%. Comparing base (48371c9) to head (ce52c3f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #600      +/-   ##
==========================================
+ Coverage   72.74%   73.18%   +0.43%     
==========================================
  Files          29       30       +1     
  Lines        2077     2111      +34     
==========================================
+ Hits         1511     1545      +34     
  Misses        566      566              

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