fix: resolve unused variables in backend utils

Fixed @typescript-eslint/no-unused-vars errors in:
- starter.ts: removed unused error variables (2 fixes)
- auto-ssl-setup.ts: removed unused error variable (1 fix)
- ssh-key-utils.ts: removed unused error variables (3 fixes)
- user-crypto.ts: removed unused error variables (5 fixes)
- data-crypto.ts: removed unused plaintextFields and error variables (2 fixes)
- simple-db-ops.ts: removed unused parameters _userId and _tableName (2 fixes)

Total: 15 unused variable errors fixed

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

src/backend/utils/ssh-key-utils.ts

@@ -49,7 +49,7 @@ function detectKeyTypeFromContent(keyContent: string): string {
       }
 
       return "ssh-rsa";
-    } catch (error) {
+    } catch {
       return "ssh-rsa";
     }
   }
@@ -240,7 +240,7 @@ export function parseSSHKey(
             } else {
               publicKey = "";
             }
-          } catch (error) {
+          } catch {
             publicKey = "";
           }
 
@@ -318,7 +318,7 @@ export function detectKeyType(privateKeyData: string): string {
       return "unknown";
     }
     return parsedKey.type || "unknown";
-  } catch (error) {
+  } catch {
     return "unknown";
   }
 }