fix: restore missing import to enable automatic file watching

Resolves file watcher startup failure caused by incomplete refactor in commit 3b1eb74.
The import statement for SUPPORTED_EXTENSIONS was removed during refactoring but the
code still referenced it, causing a NameError when the file watcher attempted to start.

Changes:
- Add missing import: `from ..constants import SUPPORTED_EXTENSIONS` to file_watcher_service.py
- Fixes NameError in FileWatcherService.start_monitoring() at line 151
- Restores automatic index updates when files are created, modified, or deleted
- File watcher now starts successfully and provides real-time code intelligence

Testing confirmed:
- File watcher status: "error" → "active"
- New file detection: Manual refresh required → Automatic
- File modification detection: Manual refresh required → Automatic
- System shows positive status: "File watcher is active - automatic index updates enabled"

This fix restores the auto-refresh functionality that was working prior to the
TypeScript/Zig parsing strategy refactor, eliminating the need for manual refresh
operations after file system changes.

Author: johnhuang316

src/code_index_mcp/services/file_watcher_service.py

@@ -50,6 +50,7 @@ def __init__(self):
     WATCHDOG_AVAILABLE = False
 
 from .base_service import BaseService
+from ..constants import SUPPORTED_EXTENSIONS
 
 
 class FileWatcherService(BaseService):