feat(stream): honor each tenant's keepalive settings on the wheel #597

Area: streaming · follow-up to #583 story 2 · decided with Eric, 2026-09-21

stream.keepalive_interval and stream.keepalive_buckets are per tenant, since every tenant folder carries a full config.json. The keepalive wheel (stream.Heartbeater) is still one per process. Story 2 keeps the single wheel and runs it at the shortest keepalive_interval among the serving tenants: the interval is an upper bound on how long a quiet stream goes unwritten, so the shortest one satisfies every tenant.

That is a stopgap. One tenant sets the cadence for all of them: a tenant asking for 1 second makes every tenant's streams write a keepalive every second. The wheel should respect each tenant's own interval and bucket count.

Options to weigh:

• a wheel per tenant, started with the tenant's first stream and stopped after its last
• one wheel per distinct (interval, buckets) pair, shared by the tenants that use it

Either way the stream handler has to know which tenant a connection belongs to. It does not read the request's tenant today; story 5 (hub subscriptions keyed by tenant) is where it starts to, so this fits after story 5.

Done when a stream opened under tenant A is kept alive at A's keepalive_interval, and a reload of tenant B's settings does not change A's cadence.