[grade=A] DC-082+DC-064: eliminate command injection surface + add Docker resource limits

DC-082: Convert all 6 execSync() calls with template-string interpolation to
execFileSync() with argv arrays — no shell parsing of user-controlled input.
Files: routes/ca.js (5 calls), src/docker/self-updater.js (1 call).
Also removed stale execSync imports (Codex LOW finding).

DC-064: Add --memory=512m --memory-swap=1g --cpus=1.5 to docker run in start.sh
to prevent container OOM from taking down the host.

Codex grade: A (30,783 tokens). All 1539 tests pass.

dashcaddy-api/src/docker/self-updater.js

@@ -16,7 +16,7 @@ const fsp = require('fs').promises;
 const path = require('path');
 const crypto = require('crypto');
 const os = require('os');
-const { execSync } = require('child_process');
+const { execFileSync } = require('child_process');
 const platformPaths = require('../../platform-paths');
 const isWindows = platformPaths.isWindows;
 
@@ -714,7 +714,7 @@ class SelfUpdater extends EventEmitter {
     await fsp.mkdir(destDir, { recursive: true });
     // Use tar command (available on Linux, and Git Bash on Windows)
     try {
-      execSync(`tar xzf "${tarballPath}" -C "${destDir}" --strip-components=1`, { stdio: 'pipe' });
+      execFileSync('tar', ['xzf', tarballPath, '-C', destDir, '--strip-components=1'], { stdio: 'pipe' });
     } catch (e) {
       throw new Error('Failed to extract tarball: ' + e.message);
     }