security: fix CRITICAL timing attack and command injection vulnerabilities (Issue #265)

This commit addresses 2 critical security vulnerabilities identified in the
security audit.

## CRITICAL-02: Timing Attack Vulnerability (CVSS 8.5)

**Problem:** Non-constant-time string comparison in authentication allowed
timing attacks to discover tokens character-by-character through statistical
timing analysis (estimated 24-48 hours to compromise).

**Fix:** Implemented crypto.timingSafeEqual for all token comparisons

**Changes:**
- Added AuthManager.timingSafeCompare() constant-time comparison utility
- Fixed src/utils/auth.ts:27 - validateToken method
- Fixed src/http-server-single-session.ts:1087 - Single-session HTTP auth
- Fixed src/http-server.ts:315 - Fixed HTTP server auth
- Added 11 unit tests with timing variance analysis (<10% variance proven)

## CRITICAL-01: Command Injection Vulnerability (CVSS 8.8)

**Problem:** User-controlled nodeType parameter injected into shell commands
via execSync, allowing remote code execution, data exfiltration, and network
scanning.

**Fix:** Eliminated all shell execution, replaced with Node.js fs APIs

**Changes:**
- Replaced execSync() with fs.readdir() in enhanced-documentation-fetcher.ts
- Added multi-layer input sanitization: /[^a-zA-Z0-9._-]/g
- Added directory traversal protection (blocks .., /, relative paths)
- Added path.basename() for additional safety
- Added final path verification (ensures result within expected directory)
- Added 9 integration tests covering all attack vectors

## Test Results

All Tests Passing:
- Unit tests: 11/11 ✅ (timing-safe comparison)
- Integration tests: 9/9 ✅ (command injection prevention)
- Timing variance: <10% ✅ (proves constant-time)
- All existing tests: ✅ (no regressions)

## Breaking Changes

None - All changes are backward compatible.

## References

- Security Audit: Issue #265
- Implementation Plan: docs/local/security-implementation-plan-issue-265.md
- Audit Analysis: docs/local/security-audit-analysis-issue-265.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/http-server-single-session.ts

@@ -10,6 +10,7 @@ import { SSEServerTransport } from '@modelcontextprotocol/sdk/server/sse.js';
 import { N8NDocumentationMCPServer } from './mcp/server';
 import { ConsoleManager } from './utils/console-manager';
 import { logger } from './utils/logger';
+import { AuthManager } from './utils/auth';
 import { readFileSync } from 'fs';
 import dotenv from 'dotenv';
 import { getStartupBaseUrl, formatEndpointUrls, detectBaseUrl } from './utils/url-detector';
@@ -1080,15 +1081,19 @@ export class SingleSessionHTTPServer {
       
       // Extract token and trim whitespace
       const token = authHeader.slice(7).trim();
-      
-      // Check if token matches
-      if (token !== this.authToken) {
-        logger.warn('Authentication failed: Invalid token', { 
+
+      // SECURITY: Use timing-safe comparison to prevent timing attacks
+      // See: https://github.com/czlonkowski/n8n-mcp/issues/265 (CRITICAL-02)
+      const isValidToken = this.authToken &&
+        AuthManager.timingSafeCompare(token, this.authToken);
+
+      if (!isValidToken) {
+        logger.warn('Authentication failed: Invalid token', {
           ip: req.ip,
           userAgent: req.get('user-agent'),
           reason: 'invalid_token'
         });
-        res.status(401).json({ 
+        res.status(401).json({
           jsonrpc: '2.0',
           error: {
             code: -32001,