chore(webapp): server-changes note for compute migration

Author: nicktrn

.server-changes/plan-aware-compute-migration.md

@@ -0,0 +1,10 @@
+---
+area: webapp
+type: feature
+---
+
+Gradually route a configurable percentage of free (then paid) organizations onto
+the compute backing at trigger time, with a per-org exclusion and an admin kill
+switch. Controlled by the `computeMigrationEnabled`, `computeMigrationFreePercentage`,
+and `computeMigrationPaidPercentage` feature flags and the `COMPUTE_BACKING_MAP`
+env var.