feat: Allow setting port as PORT env var

This seems important for render hosting, as described in
https://render.com/docs/web-services#port-binding.

prophet/config.py

@@ -8,6 +8,16 @@ from dotenv import load_dotenv
 _ = load_dotenv()
 
 
+@dataclass
+class AppConfig:
+    PORT: int
+
+    @classmethod
+    def from_env(cls) -> "AppConfig":
+        PORT = os.getenv("PORT", "8000")
+        return cls(PORT=int(PORT))
+
+
 @dataclass
 class AiConfig:
     API_KEY: str