feat(be-17): setup OpenTelemetry distributed tracing across Rust microservices (#866)

[MrG139]

Closes #866.

• Added tracing-opentelemetry + OTLP exporter setup in backend/src/main.rs
• Wrapped Actix App with tracing_actix_web::TracingLogger
• Instrumented send_move (backend/src/socket/game.rs) with #[tracing::instrument]
• Traces exported via OTLP to localhost:4317 (Jaeger-compatible)

Testing: cargo build passes.

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