fix: subprocess not imported in _do_apply background thread

The apply_pending_config endpoint spawns _do_apply in a background thread.
subprocess was used but not imported inside the closure, causing
NameError: name 'subprocess' is not defined on every Apply click —
silently swallowed, so containers never restarted and no error was shown.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/app.py

@@ -712,15 +712,16 @@ def apply_pending_config():
         # Run in a background thread; 0.3 s delay lets Flask send this response first.
         def _do_apply():
             import time as _time
+            import subprocess as _subprocess
             _time.sleep(0.3)
             if compose_down_args:
-                r = subprocess.run(base_cmd + compose_down_args,
-                                   capture_output=True, text=True, timeout=60)
+                r = _subprocess.run(base_cmd + compose_down_args,
+                                    capture_output=True, text=True, timeout=60)
                 if r.returncode != 0:
                     logger.error(f"docker compose down failed: {r.stderr.strip()}")
                     return
-            result = subprocess.run(base_cmd + compose_up_args,
-                                    capture_output=True, text=True, timeout=120)
+            result = _subprocess.run(base_cmd + compose_up_args,
+                                     capture_output=True, text=True, timeout=120)
             if result.returncode != 0:
                 logger.error(f"docker compose up failed: {result.stderr.strip()}")
             else: