Fix doctest compilation errors in config.rs

- Fix doctest import paths to use calendar_backend::config::CalDAVConfig
- Add proper Result handling in doctest example with ?operator
- All doctests now passing (3/3)
- Complete test coverage: 7 unit + 9 integration + 3 doc = 19 tests

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/config.rs

@@ -16,13 +16,15 @@ use base64::prelude::*;
 /// # Example
 /// 
 /// ```rust
-/// use crate::config::CalDAVConfig;
-/// 
+/// # use calendar_backend::config::CalDAVConfig;
+/// # fn example() -> Result<(), Box<dyn std::error::Error>> {
 /// // Load configuration from environment variables
 /// let config = CalDAVConfig::from_env()?;
 /// 
 /// // Use the configuration for HTTP requests
 /// let auth_header = format!("Basic {}", config.get_basic_auth());
+/// # Ok(())
+/// # }
 /// ```
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct CalDAVConfig {
@@ -72,7 +74,7 @@ impl CalDAVConfig {
     /// # Example
     /// 
     /// ```rust
-    /// use crate::config::CalDAVConfig;
+    /// # use calendar_backend::config::CalDAVConfig;
     /// 
     /// match CalDAVConfig::from_env() {
     ///     Ok(config) => {
@@ -123,7 +125,7 @@ impl CalDAVConfig {
     /// # Example
     /// 
     /// ```rust
-    /// use crate::config::CalDAVConfig;
+    /// # use calendar_backend::config::CalDAVConfig;
     /// 
     /// let config = CalDAVConfig {
     ///     server_url: "https://example.com".to_string(),