feat(activities): Aktivitäten & Timeline implementiert

Features:
- CRUD für Notizen, Anrufe, Emails, Meetings, Tasks
- Timeline View für Kontakte/Firmen/Deals
- Upcoming Tasks (nächste X Tage)
- Overdue Tasks
- Mark Complete/Reopen
- Activity Statistics (by type, today, overdue)
- Erinnerungen (reminder_at)
- Duration Tracking
- Outcome Recording

Task: #11 Aktivitäten & Timeline

src/main.ts

@@ -184,9 +184,16 @@ app.use(async (ctx, next) => {
         },
         activities: {
           "GET /api/v1/activities": "List activities",
+          "GET /api/v1/activities/stats": "Activity statistics",
           "GET /api/v1/activities/upcoming": "Upcoming tasks",
+          "GET /api/v1/activities/overdue": "Overdue tasks",
+          "GET /api/v1/activities/timeline/:type/:id": "Entity timeline",
+          "GET /api/v1/activities/:id": "Get activity",
           "POST /api/v1/activities": "Create activity",
-          "POST /api/v1/activities/:id/complete": "Complete task",
+          "PUT /api/v1/activities/:id": "Update activity",
+          "POST /api/v1/activities/:id/complete": "Mark completed",
+          "POST /api/v1/activities/:id/reopen": "Reopen activity",
+          "DELETE /api/v1/activities/:id": "Delete activity",
         },
         pipelines: {
           "GET /api/v1/pipelines": "List pipelines",