fix(update): stop a timed-out git from leaving .git/index.lock behind

subprocess.run's timeout SIGKILLs only the direct git child, so the lock
is left behind and git's helpers are orphaned. On a slow SD card the same
pull then times out on every retry and recreates the lock, so removing it
by hand does not help.

- run_git starts git in its own session, stops the group with SIGTERM
  before SIGKILL, and clears the lock if nothing owns it. Update Code's
  pull, checkout, reset and stash go through it, with a 600s pull timeout.
- Read-only calls (describe --dirty, status, the update-check fetch) set
  GIT_OPTIONAL_LOCKS=0 so a timed-out poll cannot take the lock.
- A timed-out pull returns a clear error; a rebase left by an interrupted
  update is aborted before pulling.
- The manual-fix hint says to stop the services and kill running git.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/__init__.py

@@ -35,6 +35,7 @@ from typing import Dict, Any, Optional, Tuple, Type
 from urllib.parse import urlparse, urlunparse
 logger = logging.getLogger(__name__)
 # Import new infrastructure
+from web_interface import git_index_lock
 from src.web_interface.api_helpers import (success_response, error_response,
                                            exception_error_response, validate_request_json)
 from src.web_interface.errors import ErrorCode
@@ -558,7 +559,8 @@ def get_git_version(project_dir=None):
             capture_output=True,
             text=True,
             timeout=5,
-            cwd=str(project_dir)
+            cwd=str(project_dir),
+            env=git_index_lock.read_only_env()
         )
 
         if result.returncode == 0: