DC-084/085/089/090: Quick wins batch

DC-084: Add .dockerignore (excludes __tests__/, .git/, node_modules/, coverage/)
DC-085: Replace Math.random() with crypto.randomUUID()/crypto.randomBytes() for IDs
DC-089: Add dedicated rate limiter on POST /license/activate (10 attempts/15min)
DC-090: Pin Node.js to 20.11.1-alpine3.19 + add engines field to package.json

All 1539 tests pass. ESLint: 0 errors.

dashcaddy-api/src/dns/dns-providers/rfc2136.js

@@ -10,6 +10,7 @@
 
 const { execFile } = require('child_process');
 const { promisify } = require('util');
+const crypto = require('crypto');
 const dns = require('dns');
 const os = require('os');
 const path = require('path');
@@ -117,7 +118,7 @@ class RFC2136Provider extends BaseDNSProvider {
    */
   async _runNsupdate(commands) {
     const script = commands.join('\n') + '\n';
-    const tmpFile = path.join(os.tmpdir(), `nsupdate-${Date.now()}-${Math.random().toString(36).slice(2, 8)}.cmd`);
+    const tmpFile = path.join(os.tmpdir(), `nsupdate-${crypto.randomBytes(4).toString('hex')}.cmd`);
 
     try {
       await fs.promises.writeFile(tmpFile, script, { mode: 0o600 });

dashcaddy-api/src/monitoring/health-checker.js

@@ -6,6 +6,7 @@
 
 const https = require('https');
 const http = require('http');
+const crypto = require('crypto');
 const EventEmitter = require('events');
 const fs = require('fs');
 const path = require('path');
@@ -349,7 +350,7 @@ class HealthChecker extends EventEmitter {
 
     // Create new incident
     const incident = {
-      id: `incident-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`,
+      id: `incident-${crypto.randomUUID()}`,
       serviceId,
       type,
       message,