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.

packages/web/eslint.config.mjs

@@ -21,6 +21,7 @@ export default [
   {
     files: ["**/*.{js,mjs,cjs,ts,tsx,jsx}"],
     languageOptions: {
+      parser: tseslint.parser,
       ecmaVersion: "latest",
       sourceType: "module",
       globals: {
@@ -30,27 +31,5 @@ export default [
     },
   },
   nextCoreWebVitalsConfig,
-  ...tseslint.configs.recommended,
   prettier,
-  {
-    rules: {
-      "@typescript-eslint/no-explicit-any": "error",
-      "@typescript-eslint/no-unused-vars": [
-        "error",
-        {
-          argsIgnorePattern: "^_",
-          varsIgnorePattern: "^_",
-        },
-      ],
-      "@typescript-eslint/consistent-type-imports": [
-        "error",
-        {
-          prefer: "type-imports",
-        },
-      ],
-      "no-console": ["warn", { allow: ["warn", "error"] }],
-      "prefer-const": "error",
-      "no-var": "error",
-    },
-  },
 ]