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/images.rs

@@ -38,7 +38,7 @@ pub struct GenerateRequest {
     pub references: Vec<PathBuf>,
 }
 
-fn slug(s: &str) -> String {
+pub(crate) fn slug(s: &str) -> String {
     let mut out: String = s
         .chars()
         .map(|c| if c.is_ascii_alphanumeric() { c.to_ascii_lowercase() } else { '-' })
@@ -121,7 +121,7 @@ fn pick(all: &[ImageModel], stage: &str, purpose: Option<&str>, with_reference:
 }
 
 /// Read local reference images as data URIs (fal.ai accepts them in place of URLs).
-async fn reference_uris(paths: &[PathBuf]) -> Result<Vec<String>> {
+pub(crate) async fn reference_uris(paths: &[PathBuf]) -> Result<Vec<String>> {
     let mut out = Vec::new();
     for p in paths {
         let bytes = tokio::fs::read(p).await.with_context(|| format!("reference image {}", p.display()))?;
@@ -135,7 +135,7 @@ async fn reference_uris(paths: &[PathBuf]) -> Result<Vec<String>> {
     Ok(out)
 }
 
-async fn download(url: &str) -> Result<(Vec<u8>, String)> {
+pub(crate) async fn download(url: &str) -> Result<(Vec<u8>, String)> {
     if let Some(rest) = url.strip_prefix("data:") {
         let (meta, data) = rest.split_once(',').context("malformed data URI")?;
         let ctype = meta.split(';').next().unwrap_or("image/png").to_string();
@@ -348,7 +348,7 @@ pub fn backfill_copies(app: &App) -> Result<usize> {
 }
 
 /// Downscaled copy for the model's context.
-fn preview_for_model(path: &Path) -> Option<ImageData> {
+pub(crate) fn preview_for_model(path: &Path) -> Option<ImageData> {
     let img = image::open(path).ok()?;
     let img = img.resize(1024, 1024, image::imageops::FilterType::Triangle);
     let mut buf = Vec::new();