[Grafana] Filter channel list by selected source

[sylvesterkaczmarek]

Fixes #235.

Filter the Grafana channel list using the currently selected telemetry sources. Channel options now refresh when source selections or template-expanded source values change, and stale asynchronous responses cannot overwrite newer results.

Includes frontend and backend regression coverage for source filtering, clearing filters, template variables, request races, quoted source identifiers, empty identifiers, and database errors.

[YKawesome]

I'm concerned from testing that this will skyrocket the query time of the database especially under load. Could we use the current Grafana panel time range to prevent this?

[sylvesterkaczmarek]

Addressed in 97c8e78: source-filtered channel lookups now carry the active Grafana time range, respect the query time overrides and selected ert/time field, and the backend requires and applies bounded from/to values inside the telemetry EXISTS scan. Added backend and frontend regressions. go test ./pkg/plugin, TypeScript typecheck/lint, and the focused Jest suites (13/13) pass.

[sylvesterkaczmarek]

Thanks for merging current main into the branch. The time-bounded source-filter change is still intact and the current checks are green. Could you re-review when convenient?