fix(security): replace os.popen/os.system with subprocess for command injection prevention

Replace all deprecated and unsafe command execution methods with
secure subprocess.run() calls using list arguments.

Changes:
- Replace os.popen() with subprocess.run() in tisbackup_gui.py
- Replace os.system() with subprocess.run() in tasks.py and backup_xva.py
- Add input validation for device/partition names (regex-based)
- Fix file operations to use context managers (with statement)
- Remove wildcard import from shutil
- Add timeout protection to all subprocess calls (5-30s)
- Improve error handling with proper try/except blocks

Security improvements:
- Prevent command injection vulnerabilities in USB disk operations
- Validate device paths with regex before system calls
- Use list arguments instead of shell=True to prevent injection
- Add proper error handling instead of silent failures

Code quality improvements:
- Replace deprecated os.popen() (deprecated since Python 2.6)
- Use context managers for file operations
- Remove wildcard imports for cleaner namespace
- Add comprehensive error handling and logging

Documentation:
- Add SECURITY_IMPROVEMENTS.md documenting all changes
- Document remaining security issues and recommendations
- Include testing recommendations and migration notes

BREAKING CHANGE: None - all changes are backward compatible

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

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

tasks.py

@@ -1,5 +1,6 @@
 import logging
 import os
+import subprocess
 
 from huey import RedisHuey
 
@@ -34,8 +35,12 @@ def run_export_backup(base, config_file, mount_point, backup_sections):
         return str(e)
 
     finally:
-        os.system("/bin/umount %s" % mount_point)
-        os.rmdir(mount_point)
+        # Safely unmount using subprocess instead of os.system
+        try:
+            subprocess.run(["/bin/umount", mount_point], check=True, timeout=30)
+            os.rmdir(mount_point)
+        except (subprocess.CalledProcessError, subprocess.TimeoutExpired, OSError) as e:
+            logger.error(f"Failed to unmount {mount_point}: {e}")
     return "ok"