feat(campaign): enforce attachment delivery policies

src/govoplan_campaign/backend/schema/campaign.schema.json

@@ -419,7 +419,19 @@
         "send_without_attachments": {
           "type": "boolean",
           "default": true,
-          "description": "Legacy compatibility flag. Prefer validation_policy and per-config missing_behavior for new campaigns."
+          "description": "Legacy compatibility flag. Use send_without_attachments_behavior for new campaigns."
+        },
+        "send_without_attachments_behavior": {
+          "type": "string",
+          "enum": [
+            "block",
+            "ask",
+            "drop",
+            "continue",
+            "warn"
+          ],
+          "default": "continue",
+          "description": "Campaign-wide behavior when no attachment file is resolved for an active message."
         },
         "zip": {
           "$ref": "#/$defs/zip_collection_config",