fix: 恢复所有迁移文件以修复校验和错误

恢复了以下迁移文件到原始状态：
- 004_add_redeem_code_notes.sql
- 005_schema_parity.sql
- 006_fix_invalid_subscription_expires_at.sql
- 007_add_user_allowed_groups.sql
- 008_seed_default_group.sql
- 009_fix_usage_logs_cache_columns.sql
- 010_add_usage_logs_aggregated_indexes.sql
- 011_remove_duplicate_unique_indexes.sql
- 012_add_user_subscription_soft_delete.sql
- 013_log_orphan_allowed_groups.sql
- 014_drop_legacy_allowed_groups.sql
- 015_fix_settings_unique_constraint.sql
- 016_soft_delete_partial_unique_indexes.sql
- 018_user_attributes.sql
- 019_migrate_wechat_to_attributes.sql
- 024_add_gemini_tier_id.sql

数据库迁移文件不应在应用后修改，即使只是注释。

backend/migrations/008_seed_default_group.sql

@@ -1,4 +1,4 @@
--- Seed a default group for fresh installs.
-INSERT INTO groups (name, description, created_at, updated_at)
-SELECT 'default', 'Default group', NOW(), NOW()
-WHERE NOT EXISTS (SELECT 1 FROM groups);
+-- Seed a default group for fresh installs.
+INSERT INTO groups (name, description, created_at, updated_at)
+SELECT 'default', 'Default group', NOW(), NOW()
+WHERE NOT EXISTS (SELECT 1 FROM groups);