Poll lifecycle for Web channel calls created via POST /voice/calls/web-new.

Resolution order:

1. active — session found in Redis (multi-pod safe)
2. ended — session cleaned up; matching row in voice_conversations
3. 503 — conversation row exists but end time not yet persisted (startup/teardown window)

Requires tenant scope: pass astraTenantId and/or watiTenantId.
With API key auth, the gateway injects tenant_id from the key context.