Update README with features, API, and dev workflow

[aronreisx]

Updates README.md to reflect the current state of the service, including core features, available endpoints, configuration, and developer workflow. Adds concise Quick Start instructions and curl examples to reduce setup friction.

• Changes

  • Added Quick Start with .env variables and make server
  • Documented health/readiness and accounts endpoints with curl examples
  • Listed key Make targets for SQLC, migrations, lint, and tests
  • Summarized architecture directories and non-functional aspects (migrations, linting, security scans, distroless image)
  • Noted that transfers/entries exist in the store layer but are not exposed via HTTP yet

• Rationale

  • Aligns documentation with implemented code and tooling
  • Provides an actionable path for running locally and via Docker Compose
  • Clarifies boundaries of the current API surface and future work

• References

  • Endpoint registration: api/server.go:28-35
  • Account handlers: api/account.go:16-36 , api/account.go:42-61 , api/account.go:68-87
  • Readiness gating: main.go:60-66 , api/server.go:49-76
  • Migrations runner: util/migration.go:15-73
  • Transfer transaction (store-layer only): db/sqlc/store.go:64-113
  • Make targets: Makefile:26-70

• Follow-ups

  • Expose transfer/entry operations via HTTP endpoints
  • Expand API docs when new endpoints are added
  • Consider adding a minimal OpenAPI spec once the API grows

• Testing

  • Documentation-only change; no code impacts or breaking changes
  • Verified commands referenced exist: make server , make sqlc-generate , make db-migrate-up , make lint-check , make test-run