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/core/config.py

@@ -0,0 +1,70 @@
+from __future__ import annotations
+
+from functools import lru_cache
+from os import environ
+from pathlib import Path
+
+from pydantic import Field
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+SERVER_DIR = Path(__file__).resolve().parents[3]
+ROOT_DIR = SERVER_DIR.parent
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(
+        env_file=(ROOT_DIR / ".env", SERVER_DIR / ".env"),
+        env_file_encoding="utf-8",
+        extra="ignore",
+    )
+
+    app_name: str = Field(default="X-Financial Server", alias="APP_NAME")
+    app_env: str = Field(default="local", alias="APP_ENV")
+    app_debug: bool = Field(default=True, alias="APP_DEBUG")
+    setup_completed: bool = Field(default=False, alias="SETUP_COMPLETED")
+
+    company_name: str = Field(default="", alias="COMPANY_NAME")
+    company_code: str = Field(default="", alias="COMPANY_CODE")
+    admin_email: str = Field(default="", alias="ADMIN_EMAIL")
+
+    web_host: str = Field(default="127.0.0.1", alias="WEB_HOST")
+    web_port: int = Field(default=5173, alias="WEB_PORT")
+    app_host: str = Field(default="127.0.0.1", alias="SERVER_HOST")
+    app_port: int = Field(default=8000, alias="SERVER_PORT")
+    api_v1_prefix: str = Field(default="/api/v1", alias="API_V1_PREFIX")
+
+    postgres_host: str = Field(default="127.0.0.1", alias="POSTGRES_HOST")
+    postgres_port: int = Field(default=5432, alias="POSTGRES_PORT")
+    postgres_db: str = Field(default="x_financial", alias="POSTGRES_DB")
+    postgres_user: str = Field(default="postgres", alias="POSTGRES_USER")
+    postgres_password: str = Field(default="postgres", alias="POSTGRES_PASSWORD")
+
+    database_url: str | None = Field(default=None, alias="DATABASE_URL")
+    sqlalchemy_echo: bool = Field(default=False, alias="SQLALCHEMY_ECHO")
+
+    redis_url: str | None = Field(default=None, alias="REDIS_URL")
+    cors_origins: list[str] = Field(default_factory=list, alias="CORS_ORIGINS")
+    vite_api_base_url: str = Field(default="http://127.0.0.1:8000/api/v1", alias="VITE_API_BASE_URL")
+
+    @property
+    def resolved_database_url(self) -> str:
+        if self.database_url:
+            return self.database_url
+
+        return (
+            f"postgresql+psycopg://{self.postgres_user}:{self.postgres_password}"
+            f"@{self.postgres_host}:{self.postgres_port}/{self.postgres_db}"
+        )
+
+
+@lru_cache
+def get_settings() -> Settings:
+    return Settings()
+
+
+def refresh_settings(updated_values: dict[str, str]) -> Settings:
+    for key, value in updated_values.items():
+        environ[key] = value
+
+    get_settings.cache_clear()
+    return get_settings()