feat(server): add configurable worker count to prevent single-worker blocking

[r266-tech]

Summary

Adds a --workers CLI flag and server.workers config option to allow running the OpenViking HTTP server with multiple uvicorn worker processes.

Problem

As reported in #464, OpenViking runs with a single uvicorn worker by default. A single slow or blocking request (e.g. a heavy commit operation) can stall the entire server, causing even lightweight endpoints like /health to become unresponsive. This is a serious availability issue for production deployments.

Solution

• Add workers: int = 1 field to ServerConfig
• Add --workers CLI argument to openviking-server
• Support server.workers in ov.conf config file
• When workers > 1, use uvicorn's factory mode with an import string so each worker process can independently initialize the application
• Default remains 1 to preserve existing single-worker behavior

Configuration

# CLI
openviking-server --workers 4

# ov.conf
{
  "server": {
    "workers": 4
  }
}

Changes

• openviking/server/config.py: Add workers field to ServerConfig and load from config
• openviking/server/bootstrap.py: Add --workers CLI arg, multi-worker uvicorn launch with factory mode

Notes

• Minimal, backward-compatible change — default behavior (single worker) is unchanged
• Multi-worker mode uses uvicorn's factory=True with the app import string, which is the standard approach for multi-process uvicorn
• For more advanced concurrency needs (e.g. wrapping blocking I/O in run_in_executor), that can be addressed in follow-up PRs

Closes #464