test(e2e,verify): enhance live API verification and dynamic port binding in E2E runner

- Update verify_counts.py to validate SQLite database records and live Go REST API endpoints directly
- Harden E2E test runner with dynamic free port binding (get_free_port) to prevent port collisions
- Add exact regex matching for dynamic (ƒ) and static (○) route table allocations
- Ignore root build binary artifacts in backend/.gitignore

backend/.gitignore

@@ -1,5 +1,8 @@
 # Binaries and build outputs
 /bin/
+/export-content
+/api
+/seed
 
 # SQLite databases and WAL journal files
 *.db