Add core controller for discovery

[marcelveldt-traveling]

This PR moves network discovery out of mass and individual providers into a shared discovery core controller.

• adds upnp_discovery to ProviderManifest, similar to existing mdns_discovery
• centralizes shared Zeroconf browsing, SSDP discovery, and _mass._tcp server advertisement
• routes SSDP discovery results back into providers through a common callback
• moves the UPnP broadcast/network-scan option to the discovery core config
• updates the existing DLNA and Roku providers to use the new discovery flow
• narrows DLNA SSDP matching to the MediaRenderer device target instead of ssdp:all
• adds a README and tests for the new controller

The result is a cleaner discovery model that new UPnP/DLNA-based providers can subscribe to without re-implementing their own discovery loop.

[github-actions]

🔒 Dependency Security Report

✅ No dependency changes detected in this PR.