@spec api_key(atom()) :: String.t() | nil

Gets an API key for a specific provider.

Handles runtime resolution of system environment variables.

@spec api_keys() :: map()

Gets all API keys as a map, resolving system environment variables.

@spec cache_config() :: keyword()

Gets the caching configuration.

@spec cache_ttl() :: pos_integer()

Gets the cache TTL (time-to-live) in milliseconds.

@spec get(atom() | [atom()], term()) :: term()

Gets a configuration value for the given key.

Supports nested keys using dot notation as atoms:

• get(:api_keys)
• get([:rate_limits, :yahoo_finance])
• get(:http_timeout)

Returns the default value if the configuration is not found.

@spec http_timeout() :: pos_integer()

Gets the HTTP timeout in milliseconds.

@spec log_level() :: atom()

Gets the log level for the application.

@spec provider_config(atom()) :: map()

Gets the configuration for a specific provider.

Returns a map with provider-specific configuration including rate limits, API keys, base URLs, etc.

@spec rate_limit(atom()) :: pos_integer()

Gets the rate limit for a specific provider (requests per minute). This is kept for backwards compatibility but new code should use the provider-specific configurations.

@spec rate_limiting_config() :: map()

Gets the rate limiting configuration including backend and provider-specific settings.

@spec telemetry_enabled?() :: boolean()

Checks if telemetry is enabled.

@spec user_agent() :: String.t()

Gets the user agent string for HTTP requests.

@spec validate_config() :: :ok | {:error, [atom()]}

Validates that required configuration is present.

Returns :ok if all required config is present, or {:error, missing_keys} if anything is missing.