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/pyproject.toml

@@ -0,0 +1,47 @@
+[build-system]
+requires = ["setuptools>=68", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "x-financial-server"
+version = "0.1.0"
+description = "Backend service for X-Financial reimbursement and approval platform."
+readme = "README.md"
+requires-python = ">=3.11"
+dependencies = [
+  "fastapi>=0.115.0,<1.0.0",
+  "uvicorn[standard]>=0.30.0,<1.0.0",
+  "sqlalchemy>=2.0.36,<3.0.0",
+  "alembic>=1.14.0,<2.0.0",
+  "psycopg[binary]>=3.2.0,<4.0.0",
+  "pydantic-settings>=2.6.0,<3.0.0",
+  "python-dotenv>=1.0.1,<2.0.0",
+  "email-validator>=2.2.0,<3.0.0",
+]
+
+[project.optional-dependencies]
+dev = [
+  "pytest>=8.3.0,<9.0.0",
+  "httpx>=0.28.0,<1.0.0",
+  "ruff>=0.8.0,<1.0.0",
+]
+redis = [
+  "redis>=5.2.0,<6.0.0",
+]
+
+[tool.setuptools]
+package-dir = {"" = "src"}
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+pythonpath = ["src"]
+testpaths = ["tests"]
+
+[tool.ruff]
+line-length = 100
+target-version = "py311"
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "B", "UP"]