docs: document request middleware order

[220nightmore-spec]

Summary

Closes #520.

Updates docs/request-lifecycle-middleware-order.md so it reflects the current src/app.js bootstrap and the reusable route stacks in src/middleware/stacks.js.

Covered in the updated guide:

• the outer createStandardizedApp() response-envelope wrapper
• global middleware order before routes
• KYC webhook raw-body parsing before the global JSON parser
• request/correlation identifier behavior
• body-size guard behavior and invoice route-local limits
• inline route registration order
• feature router mount order and important local middleware chains
• reusable auth stack ordering
• metrics, 404, and error-handler ordering

Validation

• git diff --check
• Verified referenced source paths exist:
  • src/app.js
  • src/middleware/stacks.js
  • src/utils/routeMountRegistry.js
  • src/routes/invest.js
  • src/routes/marketplace.js
  • src/routes/retention.js

No runtime tests were run because this is a docs-only change.