chore: [BBND-1155] ESLint refactor

[rubenbermejo3]

Description

Restructure ESLint configuration across the monorepo by creating a shared @hashgraph/eslint-config package and upgrading from ESLint 8 (EOL) to ESLint 9 + typescript-eslint v8.

Motivation / Context

• ESLint 8 reached End of Life in October 2024
• Three different versions of @typescript-eslint (6.x, 7.x, 8.x) coexisted in the monorepo
• The root eslint.config.mjs was monolithic with package-specific rules using absolute paths
• Mass-payout packages had completely independent ESLint configs with duplicated dependencies
• No shared, composable ESLint configuration existed

Changes

New: packages/eslint-config/

• Shared ESLint config package with composable presets
• Base config (createBaseConfig()): TypeScript + Prettier + unused-imports
• Presets: node, react, jest, mocha, ddd, stylistic
• All ESLint plugin dependencies centralized here

Updated: Per-package eslint.config.mjs

• Each package now has its own flat config extending from @hashgraph/eslint-config
• Configs compose base + environment presets + local overrides:
  • packages/ats/contracts: base + node + mocha
  • packages/ats/sdk: base + node + jest + explicit-return-type
  • apps/ats/web: base + react + jest
  • apps/mass-payout/backend: base + node + stylistic + ddd + jest
  • apps/mass-payout/frontend: base + react + jest
  • packages/mass-payout/contracts: base + node + stylistic + ddd + mocha
  • packages/mass-payout/sdk: base + node + stylistic + ddd + jest

Updated: Root config

• Minimized to only ignore packages/** and apps/** (each package lints itself)

Updated: Dependencies

• Upgraded eslint to ^9.24.0
• Unified on typescript-eslint v8 (replaces separate parser + plugin)
• Removed 11+ ESLint devDeps from root (moved to shared config)
• Removed ~20 ESLint devDeps from mass-payout backend/frontend
• Added @hashgraph/eslint-config: "*" to all packages

Updated: Scripts

• Normalized all packages: "lint": "eslint . --cache", "lint:fix": "eslint . --fix --cache"
• Root scripts: lint:js runs all workspaces, ats:lint:js and mass-payout:lint for product-specific linting
• Added lint steps to CI workflows

Cleanup

• Deleted packages/ats/sdk/.eslintignore (ignores moved to config)
• Added .npmrc with legacy-peer-deps=true for pre-existing Hardhat peer dep conflicts

How has this been tested?

• npm run lint passes on all 7 packages without introducing new errors
• npm run ats:lint:js lints only ATS packages
• npm run mass-payout:lint lints only Mass Payout packages
• Verified no regressions: pre-existing warnings remain, no new violations

Types of changes

• Refactor (non-breaking change that restructures existing code)
• Chore (tooling, configuration, dependencies)

Checklist

• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my changes (N/A - tooling change)
• All new and existing tests pass

[hedera-eng-infrastructure]

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