## Summary
Move from a single shared S2 basin to **per-org basins** with retention
tied to the org's billing plan. Stops S2 from deleting streams out from
under live chat sessions when basin retention fires before the chat
ends, and unlocks per-org cost attribution.
OSS / s2-lite installs are unaffected: provisioning is gated by
`REALTIME_STREAMS_PER_ORG_BASINS_ENABLED` (default `false`), and the
read precedence falls back to the global basin env var when an entity
has no stamped basin.
```
basin = run.streamBasinName ?? session.streamBasinName ?? env.REALTIME_STREAMS_S2_BASIN
```
## Design
Three nullable `streamBasinName` columns (`Organization`, `TaskRun`,
`Session`) plus a provisioner that idempotently creates the basin and
reconfigures retention on plan changes. The trigger and session-create
paths stamp the org's basin onto new rows; the realtime read path picks
the basin from the entity context.
Admin routes back-fill existing orgs and force-reconfigure a single org.
## Test plan
- [x] `pnpm run typecheck --filter webapp --filter @internal/run-engine`
- [x] Backfill admin route end-to-end (provision + DB stamp + S2 basin
config).
- [x] Reconfigure on plan change (all retention tiers).
- [x] chat.agent multi-turn drives streams into the per-org basin.
- [x] Legacy fallback when entity has no stamped basin.
- [x] Provisioner is a no-op when the flag is off.
Eric Allam
386b4f65ff