feat(migration): backfill story/task sort_order from code numeric suffix

One-time Prisma migration that sets sort_order = trailing numeric part
of code for all existing stories and tasks, consistent with
parseCodeNumber (fallback = Number.MAX_SAFE_INTEGER for non-conforming
codes). PBIs are intentionally excluded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/migrations/20260514100000_backfill_story_task_sort_order_from_code/migration.sql

@@ -0,0 +1,15 @@
+-- Backfill story/task sort_order from trailing numeric part of code.
+-- Consistent with parseCodeNumber: no trailing digits → Number.MAX_SAFE_INTEGER (9007199254740991).
+-- PBIs are intentionally excluded (they keep drag-and-drop + priority ordering).
+
+UPDATE "stories"
+SET sort_order = COALESCE(
+  CAST(SUBSTRING(code FROM '[0-9]+$') AS DOUBLE PRECISION),
+  9007199254740991
+);
+
+UPDATE "tasks"
+SET sort_order = COALESCE(
+  CAST(SUBSTRING(code FROM '[0-9]+$') AS DOUBLE PRECISION),
+  9007199254740991
+);