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/schemas/reimbursement.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+from datetime import datetime
+from decimal import Decimal
+
+from pydantic import BaseModel, ConfigDict
+
+
+class ReimbursementCreate(BaseModel):
+    request_no: str
+    employee_id: str
+    title: str
+    category: str
+    amount: Decimal
+    reason: str | None = None
+
+
+class ReimbursementRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: str
+    request_no: str
+    employee_id: str
+    title: str
+    category: str
+    status: str
+    amount: Decimal
+    reason: str | None
+    created_at: datetime
+    updated_at: datetime