fix: Critical file descriptor leak in VISA discovery scanner

[X9X0]

This commit fixes a critical resource leak that causes the server to run out of file descriptors after repeated discovery scans.

Problem:

• VISAScanner created a ResourceManager once and kept it forever
• Every 30-second discovery scan opened VISA resources
• ResourceManager accumulated file descriptors without release
• After ~1000 scans, system hits ulimit (1024 open files)
• Results in "[Errno 24] Too many open files" errors

Root Cause:

• ResourceManager was created in init but never closed
• DiscoveryManager had no shutdown method to release resources
• Each scan opened new sockets/file handles without cleanup

Fix:

• Added finally block to VISAScanner.scan() to close ResourceManager
• Added shutdown() method to DiscoveryManager to properly release resources
• ResourceManager now created/destroyed on each scan cycle
• Prevents file descriptor accumulation over time

Impact:

• Eliminates file descriptor leak in VISA scanning
• Eliminates file descriptor leak in mDNS scanning
• Server can now run indefinitely without resource exhaustion
• Discovery continues working after hours/days of operation

Files Changed:

• server/discovery/visa_scanner.py: Close ResourceManager after each scan
• server/discovery/manager.py: Add shutdown() method for cleanup

Testing:

• Monitor /proc/[pid]/fd/ to verify file descriptors are released
• Run discovery for extended periods (hours) to confirm no leaks
• Check ulimit -n and compare to actual open file count