Add configuration loading and error handling

Introduces a new `config` module for loading application settings from
environment variables and a `users.toml` file. An `error` module is
added to handle application-specific errors gracefully. An
`.env.example` file is created to show available configuration options,
and `users.toml.example` provides a template for user data. The main
function now initializes the configuration and prints the server port.

server/users.toml.example

@@ -0,0 +1,11 @@
+[[user]]
+slug = "dr_mueller"
+api_key = "change-me-to-a-secure-key"
+web_password = "$2b$12$..."
+role = "doctor"
+
+[[user]]
+slug = "dr_schmidt"
+api_key = "change-me-to-a-secure-key"
+web_password = "$2b$12$..."
+role = "doctor"