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/__init__.py

@@ -0,0 +1,5 @@
+from app.models.approval import ApprovalRecord
+from app.models.employee import Employee
+from app.models.reimbursement import ReimbursementRequest
+
+__all__ = ["ApprovalRecord", "Employee", "ReimbursementRequest"]

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")

server/src/app/models/employee.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base_class import Base
+
+
+class Employee(Base):
+    __tablename__ = "employees"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    employee_no: Mapped[str] = mapped_column(String(50), unique=True, index=True)
+    name: Mapped[str] = mapped_column(String(100), index=True)
+    department: Mapped[str] = mapped_column(String(100), index=True)
+    email: Mapped[str] = mapped_column(String(255), unique=True, index=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+    reimbursement_requests = relationship("ReimbursementRequest", back_populates="employee")

server/src/app/models/reimbursement.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+from decimal import Decimal
+
+from sqlalchemy import DateTime, ForeignKey, Numeric, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base_class import Base
+
+
+class ReimbursementRequest(Base):
+    __tablename__ = "reimbursement_requests"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    request_no: Mapped[str] = mapped_column(String(50), unique=True, index=True)
+    employee_id: Mapped[str] = mapped_column(ForeignKey("employees.id"), index=True)
+    title: Mapped[str] = mapped_column(String(200))
+    category: Mapped[str] = mapped_column(String(50), index=True)
+    status: Mapped[str] = mapped_column(String(50), default="draft", index=True)
+    amount: Mapped[Decimal] = mapped_column(Numeric(12, 2))
+    reason: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+    employee = relationship("Employee", back_populates="reimbursement_requests")
+    approval_records = relationship("ApprovalRecord", back_populates="request", cascade="all, delete-orphan")