fix: extract datetime from slot selection ID before booking

scheduledAt was passed as raw "slot:{id}:{datetime}" — platform rejected
it. Added extract_datetime expression to pull the ISO datetime from the
third segment. Updated flow JSON with SetVariableBlock after slot input.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/messaging/flow/flow-variable.service.ts

@@ -25,10 +25,16 @@ export class FlowVariableService {
     evaluateExpression(expression: string, value: string, variables: Record<string, any>): any {
         switch (expression) {
             case 'extract_id': {
-                // Extract UUID from "doc:{uuid}:{name}" or "dept:{name}" or "slot:{id}:{datetime}"
+                // Extract second segment: "doc:{uuid}:{name}" → uuid, "dept:{name}" → name
                 const parts = value.split(':');
                 return parts.length >= 2 ? parts[1] : value;
             }
+            case 'extract_datetime': {
+                // Extract datetime from "slot:{doctorId}:{datetime}" → "2026-04-21T14:00:00"
+                const parts = value.split(':');
+                // Rejoin from index 2 onwards (datetime contains colons: 2026-04-21T14:00:00)
+                return parts.length >= 3 ? parts.slice(2).join(':') : value;
+            }
             case 'date_tomorrow': {
                 const d = new Date(Date.now() + 86400000);
                 return d.toISOString().split('T')[0];