chore: add pre-commit hooks, tighten ESLint, activate Sentry DSN, publish CI coverage (Phase 1)

- Install husky v9 + lint-staged: pre-commit runs eslint --fix and prettier on staged files
- Tighten ESLint base config: no-console→error, ban-ts-comment (ts-ignore banned, ts-expect-error with description allowed), reportUnusedDisableDirectives→error
- Migrate web app from deprecated `next lint` to `eslint src/` with flat config and react-hooks plugin
- Convert all 5 @ts-ignore to @ts-expect-error with descriptions, remove stale disable comments
- Add NEXT_PUBLIC_SENTRY_DSN to docker-compose.prod.yml and .env.example
- Add coverage artifact upload step to CI test job
- Pre-existing violations (102 warnings) downgraded to warn in web config for Phase 2 cleanup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -83,6 +83,13 @@ PGADMIN_PASSWORD=
 # If not set, cron endpoints are disabled.
 # CRON_SECRET=
 
+# ─── Error Tracking (Sentry) ─────────────────────────────────────────────────
+
+# Sentry DSN for client-side and server-side error reporting.
+# Create a Next.js project at https://sentry.io and copy the DSN here.
+# If not set, Sentry is disabled (SDK is installed but sends nothing).
+# NEXT_PUBLIC_SENTRY_DSN=
+
 # ─── Testing (never enable in production) ────────────────────────────────────
 
 # Disables rate limiting and session tracking during end-to-end tests.