chore: add database design rules

.cursor/rules/database-design.mdc

@@ -0,0 +1,17 @@
+---
+alwaysApply: true
+---
+
+# Database Design Guidelines
+
+1. **Database Schema**: Define all tables in `/db/migrations/0001_init.up.sql`
+
+2. **Auto-generated Code**: All Go files under `/internal/db/sqlc` are automatically generated by sqlc. **DO NOT manually modify these files!**
+
+3. **Regenerating Code**: After modifying any SQL files, run the following command to update the generated Go files:
+
+```bash
+mise run sqlc-generate
+```
+
+This will automatically update all files in `/internal/db/sqlc`.