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/models/approval.py

@@ -0,0 +1,22 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base_class import Base
+
+
+class ApprovalRecord(Base):
+    __tablename__ = "approval_records"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    request_id: Mapped[str] = mapped_column(ForeignKey("reimbursement_requests.id"), index=True)
+    approver_id: Mapped[str | None] = mapped_column(ForeignKey("employees.id"), nullable=True)
+    action: Mapped[str] = mapped_column(String(50), index=True)
+    comment: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+
+    request = relationship("ReimbursementRequest", back_populates="approval_records")