Implement Jaeger distributed tracing

[auracule007]

Summary #365

Implements backend distributed tracing with Jaeger integration using OpenTelemetry and the existing tracing stack.

Changes

• Added an OpenTelemetry tracing guard to flush spans during shutdown.
• Switched trace export to OTLP/HTTP via http://localhost:4318/v1/traces.
• Consolidated tracing/log subscriber initialization into one setup path.
• Wired tracing startup through layered observability config.
• Added default observability.tracing_endpoint.
• Added semantic HTTP request spans with status/error recording.
• Added traced Redis health checks beside DB health checks.
• Updated backend tracing documentation for issue [Backend] Implement distributed tracing with Jaeger integration #365.

Validation

• git diff --check passed for changed files.
• Full backend checks are currently blocked by unrelated existing compile/format issues outside this PR scope.

Security Notes

• No secrets introduced.
• Tracing endpoint is configurable with APP_OBSERVABILITY__TRACING_ENDPOINT.

Closes #365

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[auracule007]

@adams813 Please review this PR and check workflow