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/playwright.config.ts

@@ -8,7 +8,7 @@ export default defineConfig({
   fullyParallel: true,
   forbidOnly: !!process.env.CI,
   retries: process.env.CI ? 2 : 0,
-  workers: process.env.CI ? 1 : undefined,
+  workers: process.env.CI ? 2 : undefined,
   reporter: process.env.CI ? [["github"], ["list"]] : "list",
   use: {
     baseURL: "http://127.0.0.1:3000",
@@ -27,7 +27,7 @@ export default defineConfig({
   ],
 
   webServer: {
-    command: "bun --bun next build && bun --bun next start",
+    command: "bun run build && bun --bun next start",
     url: "http://127.0.0.1:3000",
     reuseExistingServer: !process.env.CI,
     timeout: 180000,