[AI] Add plugins/ai/models_path to override the AI models folder

data/darktableconfig.xml.in

@@ -424,6 +424,13 @@
     <shortdescription>AI models GitHub repository</shortdescription>
     <longdescription>GitHub repository for downloading AI models (format: owner/repo)</longdescription>
   </dtconfig>
+  <dtconfig>
+    <name>plugins/ai/models_path</name>
+    <type>string</type>
+    <default></default>
+    <shortdescription>custom AI models folder</shortdescription>
+    <longdescription>folder where AI models are stored. leave empty for the default location. existing models in the default location are not migrated automatically</longdescription>
+  </dtconfig>
   <dtconfig>
     <name>plugins/ai/cuda_device_id</name>
     <type min="0" max="15">int</type>

src/common/ai_models.c

@@ -428,8 +428,23 @@ static void _setup_registry(dt_ai_registry_t *registry)
   char cachedir[PATH_MAX] = {0};
   dt_loc_get_user_cache_dir(cachedir, sizeof(cachedir));
 
-  registry->models_dir = g_build_filename(g_get_user_data_dir(), 
-                                          "darktable", "models", NULL);
+  // honour plugins/ai/models_path override; "~/..." expands to $HOME
+  char *override = dt_conf_get_string("plugins/ai/models_path");
+  if(override && override[0])
+  {
+    if(override[0] == '~' && (override[1] == '/' || override[1] == '\0'))
+      registry->models_dir
+        = g_build_filename(g_get_home_dir(), override + 2, NULL);
+    else
+      registry->models_dir = g_strdup(override);
+  }
+  else
+  {
+    registry->models_dir = g_build_filename(g_get_user_data_dir(),
+                                            "darktable", "models", NULL);
+  }
+  g_free(override);
+
   registry->cache_dir = g_build_filename(cachedir, "ai_downloads", NULL);
 
   _ensure_directory(registry->models_dir);