Add IsFree subscription toggle to bypass all tier limitations

- Add IsFree boolean field to UserSubscription model
- When IsFree is true, user sees limitations_enabled=false regardless of global setting
- CheckLimit() bypasses all limit checks for IsFree users
- Add admin endpoint GET /api/admin/subscriptions/user/:user_id
- Add IsFree toggle to admin user detail page under Subscription card
- Add database migration 004_subscription_is_free
- Add integration tests for IsFree functionality
- Add task kanban categorization documentation

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

migrations/004_subscription_is_free.down.sql

@@ -0,0 +1,2 @@
+-- Remove is_free column from subscription_usersubscription table
+ALTER TABLE subscription_usersubscription DROP COLUMN IF EXISTS is_free;