feat: govern attachment exceptions and ownership transfers

src/govoplan_campaign/backend/messages/models.py

@@ -33,6 +33,7 @@ class MessageIssue(BaseModel):
     message: str
     behavior: str | None = None
     source: str | None = None
+    details: dict[str, object] = Field(default_factory=dict)
 
 
 class MessageAddress(BaseModel):
@@ -49,6 +50,7 @@ class MessageAttachmentSummary(BaseModel):
     label: str | None = None
     status: str
     behavior: str | None = None
+    missing_policy: dict[str, object] | None = None
     required: bool
     allow_multiple: bool
     zip_enabled: bool