Fix security vulnerabilities, async bugs, and timing issues in FPC worker

[Copilot]

Code review identified critical bugs in cache worker: missing radix parameters causing potential octal interpretation, async forEach not awaiting ESI fragments, and KV sync timing comparing milliseconds to seconds.

Security & Correctness

parseInt radix - Added explicit base-10 to 7 calls preventing octal/hex misinterpretation:

// Critical in Content-Length checks and cache versioning
parseInt(response.headers.get('age') || '0', 10)

Null header access - Added fallback values at 3 header access points to prevent runtime errors.

Hostname validation - Added regex for OTHER_HOST to prevent DNS rebinding:

OTHER_HOST.match(/^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?(\.[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?)*(:[0-9]{1,5})?$/i)

Bug Fixes

ESI async processing - Replaced forEach(async ...) with Promise.all + map. Previous code didn't await, causing replacements to fail silently:

// Now properly awaits all text() calls in parallel
const textPromises = results.map(async (result, ind) => {
  try {
    return result.status === 200 ? await result.text() : "<!--ESI not 200-->";
  } catch (error) {
    return "<!--ESI error-->";
  }
});
const texts = await Promise.all(textPromises);

KV config sync timing - Fixed comparison === 360 (never true) to > 360000 (6 minutes in ms). Moved timestamp reset before sync to prevent interval drift.

Code Quality

• Fixed variable typo: R2_CAHE_LOGGEDIN_USERS → R2_CACHE_LOGGED_IN_USERS
• Removed dead code: unused options variable in cache bypass check
• Fixed 10+ comment typos (simultaneously, received, instead, etc.)
• Removed "nodejent" from package.json keywords

Documentation

• Created SECURITY.md covering environment variables, ESI risks, and mitigation strategies
• Fixed README typos: Cockies→Cookies, Chache→Cache, malformed HTML tags
• Created IMPROVEMENTS.md with detailed change log

CodeQL: 0 alerts. All changes backward compatible.

Original prompt

Review and suggest not improvements

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