feat(contracts): add notification.requested event schema (GOO-173)

Phase 1 RFC-004 prep — define the event contract that the notifications
module will publish through the Phase 0 outbox + Redis Streams backbone.

- Add notification.requested.schema.json (JSON Schema 2020-12) covering
  notificationId, userId, category, template, params, channels, locale,
  priority, dedupeKey, requestedAt
- Add NotificationRequestedPayload + supporting union types
  (NotificationCategory, NotificationChannel, NotificationLocale,
  NotificationPriority) to @goodgo/contracts-events barrel
- Register 'notification.requested' in KNOWN_EVENT_TYPES

Pure contract addition; no producer/consumer wiring yet (follow-up commits
in this branch will introduce the publisher refactor + BullMQ worker).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

libs/contracts/events/schemas/notification.requested.schema.json

@@ -0,0 +1,81 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://goodgo.vn/schemas/events/notification.requested.schema.json",
+  "title": "notification.requested",
+  "description": "Emitted when a domain action requests delivery of a user-facing notification. Consumed by the notifications worker pool, which fans out to email / SMS / Zalo OA / FCM / in-app channels per user preferences.",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "notificationId",
+    "userId",
+    "category",
+    "template",
+    "params",
+    "channels",
+    "requestedAt"
+  ],
+  "properties": {
+    "notificationId": {
+      "type": "string",
+      "minLength": 1,
+      "description": "UUIDv7 of the persisted Notification aggregate (also used as idempotency key by the consumer)."
+    },
+    "userId": {
+      "type": "string",
+      "minLength": 1
+    },
+    "category": {
+      "type": "string",
+      "enum": [
+        "auth",
+        "kyc",
+        "listing",
+        "payment",
+        "subscription",
+        "inquiry",
+        "lead",
+        "agent",
+        "residential",
+        "system"
+      ]
+    },
+    "template": {
+      "type": "string",
+      "minLength": 1,
+      "description": "Template key resolved by TemplateService (e.g. 'listing.approved.email.vi')."
+    },
+    "params": {
+      "type": "object",
+      "description": "Template substitution parameters. Shape depends on template; consumer is responsible for validation against the template manifest.",
+      "additionalProperties": true
+    },
+    "channels": {
+      "type": "array",
+      "minItems": 1,
+      "items": {
+        "type": "string",
+        "enum": ["email", "sms", "zalo", "fcm", "in_app"]
+      },
+      "description": "Channels requested by the producer. The consumer further filters by user notification preferences before dispatch."
+    },
+    "locale": {
+      "type": "string",
+      "enum": ["vi", "en"],
+      "description": "Preferred locale; falls back to user's stored locale, then 'vi'."
+    },
+    "priority": {
+      "type": "string",
+      "enum": ["low", "normal", "high", "critical"],
+      "default": "normal"
+    },
+    "dedupeKey": {
+      "type": "string",
+      "minLength": 1,
+      "description": "Optional consumer-side dedupe key (collapses duplicate logical notifications inside the BullMQ queue, in addition to envelope-level eventId dedupe)."
+    },
+    "requestedAt": {
+      "type": "string",
+      "format": "date-time"
+    }
+  }
+}