feat(campaign): govern attachment reuse

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

@@ -372,6 +372,10 @@
           ],
           "default": "ask"
         },
+        "reuse_policy": {
+          "$ref": "#/$defs/attachment_reuse_policy",
+          "description": "Controls whether resolving the same source file more than once is allowed, warned, reviewed, or blocked, with an optional same-recipient or same-message allowance."
+        },
         "residual_files": {
           "$ref": "#/$defs/residual_file_disposition"
         }
@@ -1458,6 +1462,25 @@
       },
       "additionalProperties": false
     },
+    "attachment_reuse_policy": {
+      "type": "object",
+      "properties": {
+        "action": {
+          "type": "string",
+          "enum": ["allow", "warn", "review", "block"],
+          "default": "allow",
+          "description": "Action when one resolved source file is used more than once outside the configured allowance. Review creates an explicit, reasoned review decision."
+        },
+        "allow_within": {
+          "type": "string",
+          "enum": ["none", "same_recipient", "same_message"],
+          "default": "none",
+          "description": "Optional exception that permits reuse confined to one recipient or one built message."
+        }
+      },
+      "additionalProperties": false,
+      "default": {"action": "allow", "allow_within": "none"}
+    },
     "residual_file_disposition": {
       "type": "object",
       "properties": {

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

@@ -61,6 +61,16 @@
     }
   ],
   "fields": {
+    "/attachments/reuse_policy/action": {
+      "label": "Duplicate-file action",
+      "control": "select",
+      "description": "Allow and record, warn, require a reasoned review decision, or block repeated use of the same resolved file."
+    },
+    "/attachments/reuse_policy/allow_within": {
+      "label": "Allowed reuse boundary",
+      "control": "select",
+      "description": "Optionally exempt reuse confined to one recipient or one built message."
+    },
     "/attachments/global[]/message_filename_template": {
       "label": "Direct attachment filename",
       "control": "text",