Refactor handlers.rs into modular structure for better maintainability

- Split 1921-line handlers.rs into focused modules:
  - handlers/auth.rs: Authentication handlers (login, verify_token, get_user_info)
  - handlers/calendar.rs: Calendar management (create_calendar, delete_calendar)
  - handlers/events.rs: Event operations (CRUD operations, fetch events)
  - handlers/series.rs: Event series operations (recurring events management)
- Main handlers.rs now serves as clean re-export module
- All tests passing (14 integration + 7 unit + 3 doc tests)
- Maintains backward compatibility with existing API routes
- Improves code organization and separation of concerns

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

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

backend/src/handlers/calendar.rs

@@ -0,0 +1,71 @@
+use axum::{
+    extract::State,
+    http::HeaderMap,
+    response::Json,
+};
+use std::sync::Arc;
+
+use crate::{AppState, models::{ApiError, CreateCalendarRequest, CreateCalendarResponse, DeleteCalendarRequest, DeleteCalendarResponse}};
+use crate::calendar::CalDAVClient;
+
+use super::auth::{extract_bearer_token, extract_password_header};
+
+pub async fn create_calendar(
+    State(state): State<Arc<AppState>>,
+    headers: HeaderMap,
+    Json(request): Json<CreateCalendarRequest>,
+) -> Result<Json<CreateCalendarResponse>, ApiError> {
+    let token = extract_bearer_token(&headers)?;
+    let password = extract_password_header(&headers)?;
+
+    // Validate request
+    if request.name.trim().is_empty() {
+        return Err(ApiError::BadRequest("Calendar name is required".to_string()));
+    }
+
+    // Create CalDAV config from token and password
+    let config = state.auth_service.caldav_config_from_token(&token, &password)?;
+    let client = CalDAVClient::new(config);
+
+    // Create calendar on CalDAV server
+    match client.create_calendar(&request.name, request.description.as_deref(), request.color.as_deref()).await {
+        Ok(_) => Ok(Json(CreateCalendarResponse {
+            success: true,
+            message: "Calendar created successfully".to_string(),
+        })),
+        Err(e) => {
+            eprintln!("Failed to create calendar: {}", e);
+            Err(ApiError::Internal(format!("Failed to create calendar: {}", e)))
+        }
+    }
+}
+
+pub async fn delete_calendar(
+    State(state): State<Arc<AppState>>,
+    headers: HeaderMap,
+    Json(request): Json<DeleteCalendarRequest>,
+) -> Result<Json<DeleteCalendarResponse>, ApiError> {
+    let token = extract_bearer_token(&headers)?;
+    let password = extract_password_header(&headers)?;
+
+    // Validate request
+    if request.path.trim().is_empty() {
+        return Err(ApiError::BadRequest("Calendar path is required".to_string()));
+    }
+
+    // Create CalDAV config from token and password
+    let config = state.auth_service.caldav_config_from_token(&token, &password)?;
+    let client = CalDAVClient::new(config);
+
+    // Delete calendar on CalDAV server
+    match client.delete_calendar(&request.path).await {
+        Ok(_) => Ok(Json(DeleteCalendarResponse {
+            success: true,
+            message: "Calendar deleted successfully".to_string(),
+        })),
+        Err(e) => {
+            eprintln!("Failed to delete calendar: {}", e);
+            Err(ApiError::Internal(format!("Failed to delete calendar: {}", e)))
+        }
+    }
+}