feat: add request tracing instrumentation

[Chucks1093]

Closes #75

Summary

• add a lightweight tracing subsystem built on AsyncLocalStorage
• add request middleware that creates API spans and returns x-trace-id / x-request-id headers
• add instrumented contract repository and Stellar RPC modules so DB and RPC work emit spans too
• refactor app bootstrap into testable modules and keep the existing endpoints working
• document the tracing flow in README and docs/backend/request-tracing.md

Verification

• npm ci
• npm run build
• npm test -- --runInBand

Notes

• the repo's lint script currently points to eslint, but eslint is not installed in package.json, so CI-relevant validation remains build plus test

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[mikewheeleer]

Please resolve the conflicts

[Chucks1093]

Okay on it