Implement many-to-many TODO user assignments

- Create todo_users junction table for many-to-many relationship
- Add migration to create todo_users table and migrate existing data
- Update validators to accept assignedUserIds array instead of assignedTo
- Update todo service to handle multiple user assignments
- Fetch and return assigned users with each TODO

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

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

src/validators/crm.validators.js

@@ -63,7 +63,7 @@ export const createTodoSchema = z.object({
   description: z.string().max(1000).optional(),
   projectId: z.string().uuid('Neplatný formát project ID').optional().or(z.literal('')),
   companyId: z.string().uuid('Neplatný formát company ID').optional().or(z.literal('')),
-  assignedTo: z.string().uuid('Neplatný formát user ID').optional().or(z.literal('')),
+  assignedUserIds: z.array(z.string().uuid('Neplatný formát user ID')).optional(),
   status: z.enum(['pending', 'in_progress', 'completed', 'cancelled']).optional(),
   priority: z.enum(['low', 'medium', 'high', 'urgent']).optional(),
   dueDate: z.string().optional().or(z.literal('')),
@@ -74,7 +74,7 @@ export const updateTodoSchema = z.object({
   description: z.string().max(1000).optional(),
   projectId: z.string().uuid('Neplatný formát project ID').optional().or(z.literal('').or(z.null())),
   companyId: z.string().uuid('Neplatný formát company ID').optional().or(z.literal('').or(z.null())),
-  assignedTo: z.string().uuid('Neplatný formát user ID').optional().or(z.literal('').or(z.null())),
+  assignedUserIds: z.array(z.string().uuid('Neplatný formát user ID')).optional(),
   status: z.enum(['pending', 'in_progress', 'completed', 'cancelled']).optional(),
   priority: z.enum(['low', 'medium', 'high', 'urgent']).optional(),
   dueDate: z.string().optional().or(z.literal('').or(z.null())),