chore(stellar-wave): close #114, #115, #116, #117 (eulami)

[eulami]

Summary

Closes the four GitHub issues assigned to @eulami as part of the
Stellar Wave Program (7th wave), bundling independently-mergeable
improvements to developer experience and CI reliability:

• Add CI caching for node_modules and cargo registry/target UnityChainxx/StellarHunts#114 Cache Scarb / cargo registry and node_modules in CI.
• Add CODEOWNERS UnityChainxx/StellarHunts#115 Add a .github/CODEOWNERS file documenting module owners.
• Add a Makefile for common devops tasks UnityChainxx/StellarHunts#116 Add a top-level Makefile so new contributors can use
  make exclusively.
• backend/README.md is referenced in CONTRIBUTING.md but does not exist UnityChainxx/StellarHunts#117 Expand backend/README.md with an explicit .env
  contract, scripts table, and run-modes table.

Changes

ci(workflow): cache scarb/cargo registry and node_modules (closes UnityChainxx#114)

actions/cache@v4 is added to both build and test jobs in
.github/workflows/build.yml. The cache covers:

• ~/.cargo/registry, ~/.cargo/git — Cargo registry (literal
  wording from the issue)
• ~/.cache/scarb, ~/.local/share/scarb, ~/.scarb — Scarb caches
• onchain/target — Scarb build output (shared between jobs)
• **/node_modules — forward-compatible for future JS steps

The cache key hashes onchain/Scarb.lock, onchain/Scarb.toml, and
any package-lock.json so dependency changes invalidate the entry.

A top-level concurrency: { cancel-in-progress: true } block cancels
superseded runs to save runner minutes.

chore(governance): add CODEOWNERS listing per-module owners (closes UnityChainxx#115)

New .github/CODEOWNERS:

• Default owners @UnityChainxx @eulami for everything.
• Per-area overrides for /frontend, /backend, /onchain,
  /.github, /Makefile, /package.json, and /docs.

The last matching rule takes precedence per GitHub CODEOWNERS
semantics.

chore(devops): add top-level Makefile for common tasks (closes UnityChainxx#116)

29 targets, all .PHONY:

• install: install, install-frontend, install-backend
• dev: dev (parallel via make -j2), dev-backend,
  dev-frontend, dev-stop (scoped pkill -f to the repos binaries)
• test: test, test-frontend, test-backend,
  test-backend-e2e
• contracts: contracts.build, contracts.test, contracts.fmt,
  contracts.fmt-check
• lint: lint, lint-frontend, lint-backend
• fmt: fmt, fmt-backend (frontend has no Prettier dep — see
  comment in the file)
• build: build, build-frontend, build-backend
• clean: clean, clean-{frontend,backend,onchain}
• ci: mirrors the GH Actions pipeline locally

make help renders the full list using a self-documenting awk block.

docs(backend): expand README with .env contract, scripts, and run modes (closes UnityChainxx#117)

Augmented the existing backend README with:

1. Install: explicit steps via make install-backend or manual
   npm install.
2. Environment Variables (.env contract): reference table
   (Required / Default / Source / Purpose) for every variable read
   by backend/config/app.config.ts and database.config.ts, plus
   JWT_SECRET and STARKNET_MODE.
3. Scripts: table for every script in backend/package.json.
4. Run Modes: table explaining start:dev / start:debug /
   start:prod.

Testing

• python3 -c "import yaml; yaml.safe_load(...)" — YAML valid.
• make help — renders all 29 targets.
• make -n <target> — dry-run confirms grouping for install,
  dev-all, contracts.test, lint, ci.

Commits

• 3cfd016 ci(workflow): cache scarb/cargo registry and node_modules
• 66e82c4 chore(devops): add top-level Makefile for common tasks
• 63387d1 chore(governance): add CODEOWNERS listing per-module owners
• fd06a39 docs(backend): expand README with .env contract, scripts, and run modes

Related Issues

Closes UnityChainxx#114
Closes UnityChainxx#115
Closes UnityChainxx#116
Closes UnityChainxx#117

[eulami]

Superseded by UnityChainxx#125 which targets the upstream repo where issues UnityChainxx#114, UnityChainxx#115, UnityChainxx#116, UnityChainxx#117 actually live. Closing this fork PR to avoid confusion.