feat: implemented account system

.env.example

@@ -1,5 +1,10 @@
-# Set a password to protect the app. Leave empty to disable auth.
-PASSWORD=
+# Admin account seeded on first run (if no users exist yet).
+ADMIN_USERNAME=admin
+ADMIN_PASSWORD=changeme
+
+# Secret key for Flask sessions. Set a fixed value so sessions survive restarts.
+# Generate one with: python -c "import secrets; print(secrets.token_hex(32))"
+SECRET_KEY=
 
 # Host port to expose the app on.
 PORT=5000