– BACKEND: Add request‑id middleware & structured logging

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Description
Log entries currently lack a correlation ID, making tracing across services difficult.

Goal
Generate a X-Request-Id header for each incoming request (or use the inbound header if present) and include it in all tracing logs.

Tasks

1. Create a middleware that checks for X-Request-Id; if missing, generate a UUID and add it to the request extensions and response headers.
2. Configure tracing to include the request ID in each span (%{request_id} field).
3. Ensure errors and panics log the request ID.
4. Add a unit test confirming the header propagation.

Acceptance Criteria

• Every request receives a unique ID visible in logs and response headers.
• Correlated log entries can be filtered by request ID.

Dependencies
Depends on: None
Blocks: None