chore(webapp): server-changes file for per-org basin migration

Author: ericallam

.server-changes/per-org-stream-basins.md

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+---
+area: webapp
+type: feature
+---
+
+Per-org S2 basin migration (TRI-9073).
+
+- Each org gets a dedicated S2 basin with retention tied to its billing plan (free 7d / hobby 30d / pro 365d) instead of the shared 30d-blanket basin.
+- Fixes TRIGGER-CLOUD-KK noise from S2 deleting streams out from under live chat sessions when basin retention fires before the session ends.
+- Provisioned via `streamBasinProvisioner.server.ts` (S2 REST `POST /v1/basins`) at org-create time; existing orgs backfilled via the `v3.provisionStreamBasinForOrg` worker job.
+- Plan changes in `setPlan` enqueue `v3.reconfigureStreamBasinForOrg` so retention follows tier upgrades / downgrades.
+- Schema: nullable `streamBasinName` on Organization / TaskRun / Session, stamped at provisioning / trigger / session-create. Reads resolve via `run.streamBasinName ?? session.streamBasinName ?? legacy basin`.
+- Stream prefix drops the `org/{orgId}` segment for per-org basins (basin already isolates) and keeps it for the legacy basin.
+- Admin routes: `POST /admin/api/v1/stream-basins/backfill` (fan-out) and `/reconfigure` (queued job + inline tier override escape hatch).
+- Gated by `REALTIME_STREAMS_PER_ORG_BASINS_ENABLED` — OSS / s2-lite installs leave it off and keep using the single basin via the legacy fallback.