Migrate .cursorrules to .cursor/rules MDC Files

While Cursor supports a legacy root .cursorrules file, the recommended format is the .cursor/rules/ directory containing .mdc (Markdown+) files generated via /rules. Configure each .mdc file with one of four activation scopes: Always On, Auto Attached (when matching files are opened), Model Decision (based on description), or Manual (@ mention).

File: .cursor/rules/database-migrations.mdc (Scope: Auto Attached on db/migrations/**)

- Always place new SQL migration files in db/migrations/ with sequential timestamp prefixes.
- Never modify an already-committed migration file.