feat(06-01): add migration 006 uniqueness constraints with safe deduplication

- DELETE duplicate budgets keeping oldest per (user_id, start_date)
- ADD CONSTRAINT budgets_user_month_unique UNIQUE (user_id, start_date)
- DELETE duplicate categories keeping oldest per (user_id, name)
- ADD CONSTRAINT categories_user_name_unique UNIQUE (user_id, name)
- Wrapped in single BEGIN/COMMIT transaction for atomicity

supabase/migrations/006_uniqueness_constraints.sql

@@ -0,0 +1,28 @@
+-- Migration 006: Add uniqueness constraints to budgets and categories
+-- Safe deduplication runs first inside the transaction before each constraint.
+
+BEGIN;
+
+-- Deduplicate budgets: keep the oldest row per (user_id, start_date)
+DELETE FROM budgets
+WHERE id NOT IN (
+  SELECT DISTINCT ON (user_id, start_date) id
+  FROM budgets
+  ORDER BY user_id, start_date, created_at ASC
+);
+
+ALTER TABLE budgets
+  ADD CONSTRAINT budgets_user_month_unique UNIQUE (user_id, start_date);
+
+-- Deduplicate categories: keep the oldest row per (user_id, name)
+DELETE FROM categories
+WHERE id NOT IN (
+  SELECT DISTINCT ON (user_id, name) id
+  FROM categories
+  ORDER BY user_id, name, created_at ASC
+);
+
+ALTER TABLE categories
+  ADD CONSTRAINT categories_user_name_unique UNIQUE (user_id, name);
+
+COMMIT;