mergedrop: add form field + default for MERGEDROP_OUTPUT_MAX_AGE_HOURS (empty env crashed server)

apps/mergedrop/config.json

@@ -7,7 +7,7 @@
   "port": 8765,
   "id_name": "mergedrop",
   "description": "Self-hosted drag-and-drop video merger. Drop clips, reorder, and build a single MP4 with ffmpeg.",
-  "tipi_version": 3,
+  "tipi_version": 4,
   "version": "1.0.2",
   "dynamic_config": true,
   "supported_architectures": ["amd64"],
@@ -29,7 +29,15 @@
       "default": "2147483648",
       "required": false,
       "env_variable": "MERGEDROP_MAX_FILE_SIZE"
+    },
+    {
+      "type": "number",
+      "label": "Output retention (hours)",
+      "hint": "Merged output files older than this are cleaned up",
+      "default": 24,
+      "required": false,
+      "env_variable": "MERGEDROP_OUTPUT_MAX_AGE_HOURS"
     }
   ],
-  "updated_at": 1787292951819
+  "updated_at": 1787293276649
 }