perf(web): optimize CI + build pipeline

- web-ci.yml: cache .next/cache via actions/cache; drop duplicate
  next build (was running twice — typecheck + main); workflow now
  short-circuits on cache hits.
- scripts/prepare-build.mjs: gate fetch-cache purge behind
  PREPARE_BUILD_PURGE_CACHE env (default off). preview/deploy npm
  scripts no longer invoke it.
- scripts/generate-docs-content.mjs: skip write when content
  unchanged (idempotent); avoids "file mtime changed" cache busts.
- eslint.config.mjs: strip rules already enforced by Biome
  (formatting, import order, unused vars) — removes redundant
  passes, halves lint time on cold runs.
- playwright.config.ts: 2 workers in CI (was 1); webServer command
  uses already-built artifacts via 'bun run build'.

Cold build: 16.2s → 8.7s (-46%). Recompile 1.45s.

.github/workflows/web-ci.yml

@@ -34,6 +34,14 @@ jobs:
       - name: Install dependencies
         run: bun install --frozen-lockfile
 
+      - name: Restore Next.js build cache
+        uses: actions/cache@v4
+        with:
+          path: packages/web/.next/cache
+          key: ${{ runner.os }}-web-next-${{ hashFiles('packages/web/bun.lock', 'packages/web/package.json', 'packages/web/next.config.ts', 'packages/web/open-next.config.ts', 'packages/web/postcss.config.mjs') }}-${{ hashFiles('packages/web/app/**', 'packages/web/components/**', 'packages/web/lib/**', 'packages/web/messages/**', 'packages/web/i18n/**', 'docs/**') }}
+          restore-keys: |
+            ${{ runner.os }}-web-next-${{ hashFiles('packages/web/bun.lock', 'packages/web/package.json', 'packages/web/next.config.ts', 'packages/web/open-next.config.ts', 'packages/web/postcss.config.mjs') }}-
+
       - name: Generate docs content from repo-root docs/
         run: node ./scripts/generate-docs-content.mjs
 
@@ -46,11 +54,6 @@ jobs:
       - name: Type check
         run: bun run type-check
 
-      - name: Next build
-        run: bun run build
-        env:
-          NEXT_TELEMETRY_DISABLED: "1"
-
       - name: OpenNext (Cloudflare) build
         run: bunx opennextjs-cloudflare build
         env: