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/scripts/generate-docs-content.mjs

@@ -55,5 +55,18 @@ const out =
   JSON.stringify(sources, null, 2) +
   "\n"
 
-await writeFile(OUTPUT, out)
-console.log("Generated " + OUTPUT + " with " + SECTIONS.length + " HTML-compiled docs")
+async function outputIsCurrent(content) {
+  try {
+    const { readFile } = await import("node:fs/promises")
+    return (await readFile(OUTPUT, "utf8")) === content
+  } catch {
+    return false
+  }
+}
+
+if (await outputIsCurrent(out)) {
+  process.stdout.write("Docs content already current with " + SECTIONS.length + " HTML-compiled docs\n")
+} else {
+  await writeFile(OUTPUT, out)
+  process.stdout.write("Generated " + OUTPUT + " with " + SECTIONS.length + " HTML-compiled docs\n")
+}

packages/web/scripts/prepare-build.mjs

@@ -2,9 +2,12 @@ import { rmSync } from "node:fs"
 import { execSync } from "node:child_process"
 
 const buildCachePaths = [".next/cache/fetch-cache"]
+const shouldClearFetchCache = process.env.OMO_WEB_CLEAR_FETCH_CACHE === "1"
 
-for (const filePath of buildCachePaths) {
-  rmSync(filePath, { force: true, recursive: true })
+if (shouldClearFetchCache) {
+  for (const filePath of buildCachePaths) {
+    rmSync(filePath, { force: true, recursive: true })
+  }
 }
 
 execSync("node ./scripts/generate-docs-content.mjs", { stdio: "inherit" })