feat(p9): thread purpose + persist consent receipt through OutboundService

Task S2.2: OutboundService.send gains an optional purpose, passes it to the broker,
and persists the returned consentReceiptRef on IiosOutboundCommand (new nullable
column, migration outbound-consent). RouteService.execute passes 'route_forward';
calendar reminders pass 'meeting_reminder'. CMP-DENY → command BLOCKED, no send.
2 tests: receipt recorded on allow, purpose-DENY blocks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/prisma/schema.prisma

@@ -706,10 +706,11 @@ model IiosOutboundCommand {
   channelType    String
   target         String
   payload        Json
-  status         String                @default("PENDING") // PENDING | SENT | FAILED | RATE_LIMITED
-  idempotencyKey String                @unique
-  providerRef    String?
-  createdAt      DateTime              @default(now())
+  status           String              @default("PENDING") // PENDING | SENT | FAILED | RATE_LIMITED | BLOCKED
+  idempotencyKey   String              @unique
+  providerRef      String?
+  consentReceiptRef String?            // CMP consent receipt this send went out under (P9)
+  createdAt        DateTime            @default(now())
 
   attempts IiosDeliveryAttempt[]