feat(plan): override CRUD with replace-existing behavior

prisma/migrations/20260605122718_drop_wpo_unique_constraint/migration.sql

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+-- Drop the unique index on (user_id, shift_date) for work_plan_overrides.
+-- MySQL unique indexes don't ignore soft-deleted rows, which would prevent
+-- createOverride from soft-deleting an existing override and creating a new
+-- one for the same (user_id, shift_date). Application code enforces the
+-- "at most one active override per (user_id, shift_date)" invariant.
+-- The application-level check is wrapped in a transaction; see
+-- src/services/plan.service.ts:createOverride.
+--
+-- The original index was created in
+-- 20260605120000_add_work_plan/migration.sql as `uniq_wpo_user_date`.
+
+DROP INDEX `uniq_wpo_user_date` ON `work_plan_overrides`;
+
+-- Add a non-unique index for the (user_id, shift_date) lookup used by
+-- createOverride's "is there an active override for this user-day?" query
+-- and by resolveCell / resolveGrid. This is a plain index, not a unique
+-- one — multiple rows (active and soft-deleted) can share the same
+-- (user_id, shift_date) pair.
+
+CREATE INDEX `idx_wpo_user_date` ON `work_plan_overrides`(`user_id`, `shift_date`);