fix: Add request locking and caching to prevent USB conflicts

Root cause: Multiple concurrent meshcli calls were fighting for USB access,
causing "Protocol error" and 504 Gateway Timeouts.

Changes to meshcore-bridge:
- Add threading.Lock to serialize meshcli subprocess calls
- Prevent concurrent USB access that causes OSError [Errno 71]
- Reduce DEFAULT_TIMEOUT from 30s to 10s
- Add detailed logging for lock acquisition and release

Changes to main API:
- Implement 30s cache for get_channels() to reduce USB calls
- Cache invalidation after channel create/join/delete operations
- Use cached channels in /api/channels and /api/messages/updates
- Reduce HTTP timeout from 30s to 12s (10s bridge + 2s buffer)

Impact:
- Eliminates race conditions when page loads (multiple API calls)
- Prevents USB port conflicts and protocol errors
- Faster response times due to caching
- No need for manual USB resets after container restarts

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/meshcore/cli.py

@@ -10,8 +10,8 @@ from app.config import config
 
 logger = logging.getLogger(__name__)
 
-# Command timeout in seconds
-DEFAULT_TIMEOUT = 30
+# Command timeout in seconds (reduced to prevent long waits)
+DEFAULT_TIMEOUT = 12  # Reduced from 30s - bridge has 10s + 2s buffer
 RECV_TIMEOUT = 60  # recv can take longer