feat(migrations): 改进校验和错误提示和文档

- 增强迁移校验和不匹配的错误信息，提供具体解决方案
- 添加 migrations/README.md 文档说明迁移最佳实践
- 明确迁移不可变原则和正确的修改流程

backend/internal/repository/migrations_runner.go

@@ -127,7 +127,15 @@ func applyMigrationsFS(ctx context.Context, db *sql.DB, fsys fs.FS) error {
 			if existing != checksum {
 				// 校验和不匹配意味着迁移文件在应用后被修改，这是危险的。
 				// 正确的做法是创建新的迁移文件来进行变更。
-				return fmt.Errorf("migration %s checksum mismatch (db=%s file=%s)", name, existing, checksum)
+				return fmt.Errorf(
+					"migration %s checksum mismatch (db=%s file=%s)\n"+
+						"This means the migration file was modified after being applied to the database.\n"+
+						"Solutions:\n"+
+						"  1. Revert to original: git log --oneline -- migrations/%s && git checkout <commit> -- migrations/%s\n"+
+						"  2. For new changes, create a new migration file instead of modifying existing ones\n"+
+						"Note: Modifying applied migrations breaks the immutability principle and can cause inconsistencies across environments.",
+					name, existing, checksum, name, name,
+				)
 			}
 			continue // 迁移已应用且校验和匹配，跳过
 		}

backend/migrations/017_add_gemini_tier_id.sql

@@ -26,10 +26,5 @@ UPDATE accounts
 SET credentials = credentials - 'tier_id'
 WHERE platform = 'gemini'
   AND type = 'oauth'
-  AND jsonb_typeof(credentials) = 'object'
+  AND credentials->>'oauth_type' = 'code_assist';
-  AND credentials->>'tier_id' = 'LEGACY'
-  AND (
-    credentials->>'oauth_type' = 'code_assist'
-    OR (credentials->>'oauth_type' IS NULL AND credentials->>'project_id' IS NOT NULL)
-  );
 -- +goose StatementEnd

backend/migrations/README.md

@@ -0,0 +1,178 @@
+# Database Migrations
+
+## Overview
+
+This directory contains SQL migration files for database schema changes. The migration system uses SHA256 checksums to ensure migration immutability and consistency across environments.
+
+## Migration File Naming
+
+Format: `NNN_description.sql`
+- `NNN`: Sequential number (e.g., 001, 002, 003)
+- `description`: Brief description in snake_case
+
+Example: `017_add_gemini_tier_id.sql`
+
+## Migration File Structure
+
+```sql
+-- +goose Up
+-- +goose StatementBegin
+-- Your forward migration SQL here
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+-- Your rollback migration SQL here
+-- +goose StatementEnd
+```
+
+## Important Rules
+
+### ⚠️ Immutability Principle
+
+**Once a migration is applied to ANY environment (dev, staging, production), it MUST NOT be modified.**
+
+Why?
+- Each migration has a SHA256 checksum stored in the `schema_migrations` table
+- Modifying an applied migration causes checksum mismatch errors
+- Different environments would have inconsistent database states
+- Breaks audit trail and reproducibility
+
+### ✅ Correct Workflow
+
+1. **Create new migration**
+   ```bash
+   # Create new file with next sequential number
+   touch migrations/018_your_change.sql
+   ```
+
+2. **Write Up and Down migrations**
+   - Up: Apply the change
+   - Down: Revert the change (should be symmetric with Up)
+
+3. **Test locally**
+   ```bash
+   # Apply migration
+   make migrate-up
+
+   # Test rollback
+   make migrate-down
+   ```
+
+4. **Commit and deploy**
+   ```bash
+   git add migrations/018_your_change.sql
+   git commit -m "feat(db): add your change"
+   ```
+
+### ❌ What NOT to Do
+
+- ❌ Modify an already-applied migration file
+- ❌ Delete migration files
+- ❌ Change migration file names
+- ❌ Reorder migration numbers
+
+### 🔧 If You Accidentally Modified an Applied Migration
+
+**Error message:**
+```
+migration 017_add_gemini_tier_id.sql checksum mismatch (db=abc123... file=def456...)
+```
+
+**Solution:**
+```bash
+# 1. Find the original version
+git log --oneline -- migrations/017_add_gemini_tier_id.sql
+
+# 2. Revert to the commit when it was first applied
+git checkout <commit-hash> -- migrations/017_add_gemini_tier_id.sql
+
+# 3. Create a NEW migration for your changes
+touch migrations/018_your_new_change.sql
+```
+
+## Migration System Details
+
+- **Checksum Algorithm**: SHA256 of trimmed file content
+- **Tracking Table**: `schema_migrations` (filename, checksum, applied_at)
+- **Runner**: `internal/repository/migrations_runner.go`
+- **Auto-run**: Migrations run automatically on service startup
+
+## Best Practices
+
+1. **Keep migrations small and focused**
+   - One logical change per migration
+   - Easier to review and rollback
+
+2. **Write reversible migrations**
+   - Always provide a working Down migration
+   - Test rollback before committing
+
+3. **Use transactions**
+   - Wrap DDL statements in transactions when possible
+   - Ensures atomicity
+
+4. **Add comments**
+   - Explain WHY the change is needed
+   - Document any special considerations
+
+5. **Test in development first**
+   - Apply migration locally
+   - Verify data integrity
+   - Test rollback
+
+## Example Migration
+
+```sql
+-- +goose Up
+-- +goose StatementBegin
+-- Add tier_id field to Gemini OAuth accounts for quota tracking
+UPDATE accounts
+SET credentials = jsonb_set(
+    credentials,
+    '{tier_id}',
+    '"LEGACY"',
+    true
+)
+WHERE platform = 'gemini'
+  AND type = 'oauth'
+  AND credentials->>'tier_id' IS NULL;
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+-- Remove tier_id field
+UPDATE accounts
+SET credentials = credentials - 'tier_id'
+WHERE platform = 'gemini'
+  AND type = 'oauth'
+  AND credentials->>'tier_id' = 'LEGACY';
+-- +goose StatementEnd
+```
+
+## Troubleshooting
+
+### Checksum Mismatch
+See "If You Accidentally Modified an Applied Migration" above.
+
+### Migration Failed
+```bash
+# Check migration status
+psql -d sub2api -c "SELECT * FROM schema_migrations ORDER BY applied_at DESC;"
+
+# Manually rollback if needed (use with caution)
+# Better to fix the migration and create a new one
+```
+
+### Need to Skip a Migration (Emergency Only)
+```sql
+-- DANGEROUS: Only use in development or with extreme caution
+INSERT INTO schema_migrations (filename, checksum, applied_at)
+VALUES ('NNN_migration.sql', 'calculated_checksum', NOW());
+```
+
+## References
+
+- Migration runner: `internal/repository/migrations_runner.go`
+- Goose syntax: https://github.com/pressly/goose
+- PostgreSQL docs: https://www.postgresql.org/docs/