refactor: rename package scope from @caldav to @calchat

Rename all workspace packages to reflect the actual project name:
- @caldav/client -> @calchat/client
- @caldav/server -> @calchat/server
- @caldav/shared -> @calchat/shared
- Root package: caldav-mono -> calchat-mono

Update all import statements across client and server to use the new
package names. Update default MongoDB database name and logging service
identifier accordingly.

apps/server/src/ai/utils/eventFormatter.ts

@@ -1,4 +1,4 @@
-import { CalendarEvent } from "@caldav/shared";
+import { CalendarEvent } from "@calchat/shared";
 
 // German date/time formatting helpers
 export const formatDate = (d: Date) => d.toLocaleDateString("de-DE");