live_session on_mount/4 callback that drives polled trigger-coverage for every LV in the :threadline session.

Mount order

MUST run AFTER Threadline.OperatorSurface.Auth in the on_mount: list, because Auth populates :threadline_repo from opts (Pitfall 7). The canonical mount in router.ex is:

live_session :threadline,
  on_mount: [
    {Threadline.OperatorSurface.Auth, opts},
    {Threadline.OperatorSurface.Coverage.OnMount, opts}
  ]

Polling

Default 30_000 ms; configure via config :threadline, :coverage_poll_ms (global) or socket assign :threadline_coverage_poll_ms (per-mount). Floor 5_000 ms — raises ArgumentError at mount below that. The Application env is the test seam (Pitfall 13): tests set a low interval to avoid scheduling flakiness.

Error policy

On poll failure: keep the previous :threadline_coverage assign, set :threadline_coverage_error to the exception message, emit [:threadline, :health, :checked, :error] via Threadline.Telemetry, and ALWAYS reschedule (Pitfall 4 — a transient DB blip must not freeze the count). The "always reschedule" guarantee is enforced unconditionally after the try/rescue block.

PubSub forward-compat (D-30d)

Out of v1.18 scope. If real adopter pain emerges at v1.19+ scale, a runtime opt-in config :threadline, :coverage_source, {:pubsub, MyApp.PubSub} will let this hook become a no-op while a single-source GenServer broadcasts via Phoenix.PubSub. Documented here only for forward-compat navigation.