chore: migrate ESLint to flat config (v9 → v10) #36

Description

The repo still uses a legacy .eslintrc.cjs at the root. We need to migrate to ESLint flat config (eslint.config.js / .mjs), adopt the official defineConfig / globalIgnores helpers, switch to typescript-eslint v8 with parserOptions.projectService: true, and align with the requirements of ESLint v10 (which has fully removed eslintrc support).

Why

• ESLint v9 reaches EOL on 2026-08-06, so we cannot stay on the legacy config long-term.
• ESLint v10 (released Feb 2026) has removed eslintrc entirely. The current .eslintrc.cjs will be silently ignored under v10.
• parserOptions.project (current) is the old way; projectService: true reuses the TS Language Service, is faster, and avoids the documented OOM risk on larger monorepos.
• The new helpers (defineConfig, globalIgnores, extends at block level) make a shared @repo/eslint-config package straightforward when we extract it later.

Acceptance criteria

• Delete the root .eslintrc.cjs.
• Create a root eslint.config.js (or .mjs) using defineConfig and globalIgnores from eslint/config.
• Use tseslint.configs.recommendedTypeChecked (or strictTypeChecked) with parserOptions.projectService: true.
• Preserve the existing jsonc-eslint-parser handling for *.json.
• pnpm lint and pnpm lint:fix call eslint . directly (no next lint — Next 16 has removed it; the app already runs Next 16.1.6).
• apps/web migrated to eslint-config-next/typescript + core-web-vitals in flat-config form if not already.
• Upgrade ESLint to v10 and typescript-eslint to a compatible v8 patch before merging.
• CI green on pnpm lint for the whole monorepo.

Risks

• Type-checked rules are slower; keep the default disallowAutomaticSingleRunInference heuristic and add a CI-only job split if needed.
• Plugin compatibility: any plugin not yet flat-config-ready must be replaced or removed.

Related

• Documented in docs/internal/learnings/eslint/eslint-monorepo-2026.md.
• Blocks extracting packages/eslint-config (separate issue).