feat: add FastAPI backend with PostgreSQL and start script fixes

- Add server/ directory with FastAPI backend
- Fix server/start.sh to properly handle venv on Windows/Git Bash
- Add alembic migrations and pyproject.toml
- Add server tests

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/src/app/db/base.py

@@ -0,0 +1,6 @@
+from app.db.base_class import Base
+from app.models.approval import ApprovalRecord
+from app.models.employee import Employee
+from app.models.reimbursement import ReimbursementRequest
+
+__all__ = ["Base", "Employee", "ReimbursementRequest", "ApprovalRecord"]