Auto-generate SESSION_SECRET_KEY when not provided

If the env var is unset, a random key is generated at startup. This
removes a manual setup step. The README and .env.example are updated
to reflect that the variable is now optional (but recommended in
production for session persistence across restarts).

https://claude.ai/code/session_018iMt5qm4j9bk8wzytLC3E6

app/config.py

@@ -1,4 +1,5 @@
 import os
+import secrets
 from pathlib import Path
 
 from dotenv import load_dotenv
@@ -38,5 +39,5 @@ LIDARR_QUALITY_PROFILE_ID: int = int(os.getenv("LIDARR_QUALITY_PROFILE_ID", "1")
 LIDARR_ROOT_FOLDER: str = os.getenv("LIDARR_ROOT_FOLDER", "/music")
 
 # App
-SESSION_SECRET_KEY: str = _require("SESSION_SECRET_KEY")
+SESSION_SECRET_KEY: str = os.getenv("SESSION_SECRET_KEY") or secrets.token_urlsafe(32)
 DATABASE_PATH: str = os.getenv("DATABASE_PATH", "bellhop.db")