Service alerts: system-wide disruptions at call start (follow-up to #8)

[aaronbrethorst]

Follow-up to #8 / PR #23, which delivered service alerts tied to a queried stop or its routes (consumed from the REST arrivals-and-departures-for-stop references.situations).

Deferred from that iteration (spec §8): system-wide disruptions played at the very beginning of a call, before the caller selects a stop, with an affordance to hear the alert in another language.

Why it's separate

The OBA REST where API has no global "list all active alerts" endpoint — situations only appear embedded in stop/route/trip responses. Delivering reliable system-wide alerts at call start needs a global source.

Suggested approach

• Add optional config GTFS_RT_ALERTS_URL; poll & cache the GTFS-RT ServiceAlerts protobuf feed (google.golang.org/protobuf is already an indirect dep).
• Classify alerts whose informed_entity names only an agency (no route/stop/trip) as system-wide.
• In HandleVoiceStart, play any active system-wide alert before stop selection, with a language menu.
• Reuse the existing models.Situation type and formatters.FormatVoiceAlerts/FormatSMSAlerts layer built in Add GTFS-RT service alerts to SMS and voice (#8) #23 — only the source and the call-open placement are new.

Notes

• Keeps the deployment-agnostic REST path from Add GTFS-RT service alerts to SMS and voice (#8) #23 intact; the GTFS-RT feed is additive and per-deployment.
• Design/spec context: docs/superpowers/specs/2026-07-01-service-alerts-design.md §8.

[yashgoyal0110]

Hey @aaronbrethorst
can i work on it?