fix: credential-manager and crypto-utils auto-resolve data directory paths

The CREDENTIALS_FILE and ENCRYPTION_KEY_FILE env vars defaulted to
__dirname/credentials.json and __dirname/.encryption-key, which works
for the standard install (where individual files are mounted to /app/)
but breaks for deployments using a consolidated data directory at
/app/data/.

Add resolveCredentialsFile() and resolveKeyFile() helpers that:
1. Honor explicit env var if set
2. Check /app/credentials.json and /app/data/credentials.json
3. Check /app/.encryption-key and /app/data/.encryption-key
4. Default to standard path for new installs

This makes DashCaddy deployable with either pattern without requiring
custom env var configuration, which is essential for general-public
reproducibility.

dashcaddy-api/crypto-utils.js

@@ -15,8 +15,26 @@ const IV_LENGTH = 16;  // 128 bits for GCM
 const AUTH_TAG_LENGTH = 16;
 const SALT_LENGTH = 32;
 
-// Key file location (should be outside of mounted volumes for security)
-const KEY_FILE = process.env.ENCRYPTION_KEY_FILE || path.join(__dirname, '.encryption-key');
+// Resolve encryption key file path — supports both standard install (/app/.encryption-key)
+// and custom deployments with consolidated data directory (/app/data/.encryption-key)
+function resolveKeyFile() {
+  if (process.env.ENCRYPTION_KEY_FILE) {
+    return process.env.ENCRYPTION_KEY_FILE;
+  }
+  const candidates = [
+    path.join(__dirname, '.encryption-key'),
+    path.join(__dirname, 'data', '.encryption-key'),
+  ];
+  for (const candidate of candidates) {
+    if (fs.existsSync(candidate)) {
+      return candidate;
+    }
+  }
+  // No existing file — return standard path so first load creates it there
+  return candidates[0];
+}
+
+const KEY_FILE = resolveKeyFile();
 
 let encryptionKey = null;