Add keepalive API endpoints that allow streams to be kept active
without real viewers. Keepalives act as synthetic readers that
trigger on-demand publishers and prevent streams from closing when
all real viewers disconnect.
Features:
- POST /v3/paths/keepalive/add/{name} - create keepalive for path
- DELETE /v3/paths/keepalive/remove/{id} - remove keepalive by ID
- GET /v3/paths/keepalive/list - list all active keepalives
- GET /v3/paths/keepalive/get/{id} - get keepalive details
Implementation details:
- Keepalives implement defs.Reader interface
- Full authentication and authorization support
- Ownership tracking - only creator can remove keepalive
- Automatic cleanup when paths are removed
- UUID-based identification
- Asynchronous stream initialization to prevent deadlocks
