feat(02-03): add MediaAttachment model, WhatsApp normalizer, and signature verification

- Add MediaType(StrEnum) and MediaAttachment(BaseModel) to shared/models/message.py
- Add media: list[MediaAttachment] field to MessageContent
- Add whatsapp_app_secret, whatsapp_verify_token, and MinIO settings to shared/config.py
- Add normalize_whatsapp_event() to gateway/normalize.py (text, image, document support)
- Create whatsapp.py adapter with verify_whatsapp_signature() and verify_hub_challenge()
- 30 new passing tests (signature verification + normalizer)

packages/shared/shared/config.py

@@ -65,6 +65,38 @@ class Settings(BaseSettings):
         description="Slack app-level token for Socket Mode (xapp-...)",
     )
 
+    # -------------------------------------------------------------------------
+    # WhatsApp
+    # -------------------------------------------------------------------------
+    whatsapp_app_secret: str = Field(
+        default="",
+        description="WhatsApp app secret for HMAC-SHA256 webhook signature verification",
+    )
+    whatsapp_verify_token: str = Field(
+        default="",
+        description="WhatsApp webhook verification token (hub.verify_token)",
+    )
+
+    # -------------------------------------------------------------------------
+    # MinIO / Object Storage
+    # -------------------------------------------------------------------------
+    minio_endpoint: str = Field(
+        default="http://localhost:9000",
+        description="MinIO endpoint URL (S3-compatible)",
+    )
+    minio_access_key: str = Field(
+        default="minioadmin",
+        description="MinIO access key",
+    )
+    minio_secret_key: str = Field(
+        default="minioadmin",
+        description="MinIO secret key",
+    )
+    minio_media_bucket: str = Field(
+        default="konstruct-media",
+        description="MinIO bucket name for media attachments",
+    )
+
     # -------------------------------------------------------------------------
     # LLM Providers
     # -------------------------------------------------------------------------