generate_3d: 3D models with fal.ai (image, text, or image steered by text)

- New tool: image-to-3D (project file or an image attached to the chat,
  optional back/left/right views) or text-to-3D; image + prompt refines the
  image with the prompt first (the 3D models take no text with an image).
  Styles textured / geometry / lowpoly, optional PBR, face count. Runs through
  fal.ai's queue API with progress, saves GLB + preview, books the cost
  (published prices, overridable per model). Request bodies for the Hunyuan3D,
  Tripo and Trellis families; defaults [images] model_3d / model_3d_text.
- Chat card: "Open in the 3D view"; media overview: kind "model" (also
  STL/3MF/OBJ written by agents) with previews.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/config.rs

@@ -177,6 +177,19 @@ pub struct ImagesConfig {
     /// Optional fal.ai Admin key to query the account balance.
     #[serde(default, skip_serializing_if = "String::is_empty")]
     pub admin_key: String,
+    /// fal.ai model for 3D models from images (generate_3d).
+    #[serde(default = "default_model_3d")]
+    pub model_3d: String,
+    /// fal.ai model for 3D models from a text prompt only.
+    #[serde(default = "default_model_3d_text")]
+    pub model_3d_text: String,
+}
+
+fn default_model_3d() -> String {
+    "fal-ai/hunyuan3d-v3/image-to-3d".into()
+}
+fn default_model_3d_text() -> String {
+    "fal-ai/hunyuan3d-v3/text-to-3d".into()
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, Default)]
@@ -215,6 +228,8 @@ impl Default for ImagesConfig {
             models: vec![ImageModel { id: "fal-ai/flux/schnell".into(), name: Some("FLUX.1 schnell".into()), price: Some(0.003), ..Default::default() }],
             monthly_budget: None,
             admin_key: String::new(),
+            model_3d: default_model_3d(),
+            model_3d_text: default_model_3d_text(),
         }
     }
 }