feat: add request ID middleware and trace propagation (Closes #27)

[ZacLou]

Closes #27

Changes

Adds request ID middleware for trace propagation across API and gateway services:

• request_id middleware: Reads X-Request-Id header from incoming requests or generates a UUIDv4 if missing
• Response header: Injects x-request-id into all responses for client-side correlation
• Tracing span: Records request_id in the current tracing span for distributed trace propagation
• Both services: Applied to API public router and gateway router

Files changed

• crates/api/src/middleware.rs — Added request_id middleware function
• crates/api/src/main.rs — Wired into public router as outermost layer
• crates/gateway/src/routes.rs — Added inline request_id middleware, wired into gateway router

Behavior

• Client sends X-Request-Id: abc-123 → echoed back in response, recorded in logs
• Client omits header → server generates 550e8400-e29b-..., returned in response