add real-time GraphQL API with time-series queries and subscriptions
- add absinthe_phoenix for WebSocket subscription support
- create GraphQL WebSocket socket with API token auth at /socket/graphql
- add time-series queries: deviceSensors, sensorReadings, interfaceTraffic, checkResults
- add subscriptions: deviceStatusChanged, alertEvent, sensorReadingsUpdated
- wire subscription triggers into agent_channel and alerts contexts
- add org-scoped access control for sensors/interfaces via join queries
- add 17 tests covering queries, org isolation, empty data edge cases, and auth
- update GraphQL API docs page with time-series and subscription sections
Graham McIntire
72f81c572d