feat: add PostgreSQL database configuration

- Add POSTGRES_HOST, POSTGRES_USER, POSTGRES_PASSWORD settings
- Add database_url property for connection string construction
- Database: core_api on shared postgres instance

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/config.py

@@ -106,6 +106,17 @@ class Settings(BaseSettings):
     homeassistant_token: str  # Required - set HOMEASSISTANT_TOKEN in .env
     homeassistant_timeout: int = 30
 
+    # PostgreSQL Database
+    postgres_host: str  # Required - set POSTGRES_HOST in .env (e.g., localhost:5432)
+    postgres_user: str = "core_api"
+    postgres_password: str  # Required - set POSTGRES_PASSWORD in .env
+    postgres_database: str = "core_api"
+
+    @property
+    def database_url(self) -> str:
+        """Construct database URL from components"""
+        return f"postgresql://{self.postgres_user}:{self.postgres_password}@{self.postgres_host}/{self.postgres_database}"
+
     # OIDC Authentication (Authentik)
     oidc_enabled: bool = False  # Set to True to require authentication
     oidc_issuer: str = "https://auth.schweitz.net/application/o/core-api/"