docs(architecture): add "the problem this exists to solve" section

[rodrigobnogueira]

Summary

The architecture page at nest-native.dev/reference-app/architecture.html was jumping straight from a one-paragraph intro to the module graph. A reader landing there needs the "why should I care, what does this save me from figuring out" framing first — otherwise the architecture content reads as design-by-example with no clear question being answered.

This PR adds a single new H2 section between the intro and the module graph. Each bullet links to the relevant deeper section in this same doc, so the new content doubles as a TOC.

Changes

• docs/architecture.md — new ## The problem this exists to solve section. Frames the app as a commitment to specific answers for the 5 composition questions an adopter would otherwise have to design from scratch:
  • threading currentUser / currentOrganization through middleware → tRPC createContext → guards → request-scoped DI
  • transactions across services with @InjectTransaction(), including the better-sqlite3 async-adapter trap
  • post-commit side effects via the transactional outbox (atomic claim, idempotency, backoff, stuck-claim recovery)
  • keeping the typed-client contract honest via client-smoke in CI
  • the "boring scaffolding" (ESLint complexity gate, drizzle-kit, node:test + c8, the CI chain, single-image Docker for API+worker)
• Closes with a "commitment to specific answers" framing, not a "menu of options".

Modules Touched

• Tooling / CI / docs only

Public Surface (libraries)

• No use of library internals introduced.

Security Review

• Docs only.

Dependency Review

• No dependency or lockfile changes.

Migrations

• No schema changes.

Validation

• npm run ci unaffected (docs only).
• Live render — verified after merge when GitHub Pages rebuilds; the new in-page anchor links (#request-lifecycle, #authentication, #why-a-custom-syncdrizzletransactionaladapter, #outbox, #the-worker-process) all map to existing H2/H3 headings in the same doc and will be auto-generated by Jekyll.

Release Notes

• No release impact
• User-visible doc change at nest-native.dev/reference-app/architecture.html.