refactor(backend): extract download and onnx_runtime modules from captioner

captioner.rs had grown into two things: the (currently disabled)
Florence-2 captioner plus generic infrastructure the live tagger
depends on. Split the neutral parts out:

- download.rs: resilient curl downloader (resume, stall detection)
  and NuGet package extraction
- onnx_runtime.rs: shared ONNX Runtime/DirectML DLL manifest,
  provisioning, and ort init, with runtime_dir() as the single
  definition of the DLL location (kept inside the caption model dir
  so existing installs do not re-download)

Also dedupes the tagger-side copies of the DLL list and four
hardcoded caption-model path literals, and rewords tagger runtime
errors that wrongly told users to download the caption model.

src-tauri/src/onnx_runtime.rs

@@ -0,0 +1,137 @@
+//! Shared ONNX Runtime provisioning: downloading the runtime + DirectML DLLs
+//! and initializing `ort` from them.
+//!
+//! Both ONNX consumers go through here — the tagger (live) and the Florence-2
+//! captioner (backend intact, UI disabled). The DLLs are Windows/DirectML
+//! specific; a future cross-platform build needs a per-OS runtime strategy.
+
+use crate::download;
+use anyhow::Result;
+use std::path::{Path, PathBuf};
+use std::sync::Mutex;
+
+const ONNX_RUNTIME_NUGET_URL: &str =
+    "https://www.nuget.org/api/v2/package/Microsoft.ML.OnnxRuntime.DirectML/1.24.2";
+const DIRECTML_NUGET_URL: &str =
+    "https://www.nuget.org/api/v2/package/Microsoft.AI.DirectML/1.15.4";
+
+pub const ONNX_RUNTIME_DLL_FILE: &str = "onnxruntime/onnxruntime.dll";
+pub const ONNX_RUNTIME_PROVIDERS_DLL_FILE: &str = "onnxruntime/onnxruntime_providers_shared.dll";
+pub const DIRECTML_DLL_FILE: &str = "onnxruntime/DirectML.dll";
+
+/// The shared runtime DLLs, as paths relative to [`runtime_dir`].
+pub const RUNTIME_DLLS: &[&str] = &[
+    ONNX_RUNTIME_DLL_FILE,
+    ONNX_RUNTIME_PROVIDERS_DLL_FILE,
+    DIRECTML_DLL_FILE,
+];
+
+/// `(destination_file, nuget_package_url, path_inside_package)` for each DLL.
+pub const ONNX_RUNTIME_FILES: &[(&str, &str, &str)] = &[
+    (
+        ONNX_RUNTIME_DLL_FILE,
+        ONNX_RUNTIME_NUGET_URL,
+        "runtimes/win-x64/native/onnxruntime.dll",
+    ),
+    (
+        ONNX_RUNTIME_PROVIDERS_DLL_FILE,
+        ONNX_RUNTIME_NUGET_URL,
+        "runtimes/win-x64/native/onnxruntime_providers_shared.dll",
+    ),
+    (
+        DIRECTML_DLL_FILE,
+        DIRECTML_NUGET_URL,
+        "bin/x64-win/DirectML.dll",
+    ),
+];
+
+// Mutex<bool> rather than OnceLock<Result>: a failed attempt (DLL not yet
+// downloaded) must NOT be cached, or a later successful download could never
+// recover within the same app session.
+static ORT_RUNTIME_INIT: Mutex<bool> = Mutex::new(false);
+
+/// Directory the shared runtime DLLs are provisioned into.
+///
+/// Historically the DLLs were downloaded as part of the Florence-2 caption
+/// model, so they live inside that model's directory
+/// (`models/florence-2-base-ft/onnxruntime/`). The location is kept even
+/// though the tagger is now the main consumer, so existing installs don't
+/// have to re-download the runtime.
+pub fn runtime_dir(app_data_dir: &Path) -> PathBuf {
+    crate::captioner::model_dir(app_data_dir)
+}
+
+/// Initialize `ort` from the already-downloaded runtime DLL in `local_dir`.
+/// Fails if the DLL is missing — use [`provision_onnx_runtime_with_progress`]
+/// to download it first on a clean install.
+pub fn ensure_onnx_runtime(local_dir: &Path) -> Result<()> {
+    let mut initialized = ORT_RUNTIME_INIT
+        .lock()
+        .map_err(|_| anyhow::anyhow!("ONNX runtime init lock poisoned"))?;
+    if *initialized {
+        return Ok(());
+    }
+    let dll_path = local_dir.join(ONNX_RUNTIME_DLL_FILE);
+    if !dll_path.exists() {
+        anyhow::bail!("ONNX Runtime DLL is missing: {}", dll_path.display());
+    }
+    ort::environment::init_from(&dll_path)
+        .map_err(|error| anyhow::anyhow!(error.to_string()))?
+        .with_name("phokus-florence")
+        .commit();
+    *initialized = true;
+    Ok(())
+}
+
+/// Download any ONNX Runtime DLLs missing from `local_dir`, reporting per-file
+/// byte progress as `(short_label, downloaded_bytes, total_bytes)`.
+/// `total_bytes` is `None` when the server omits Content-Length. Unlike
+/// `ensure_onnx_runtime` (init only), this actually provisions the files —
+/// callers that can run on a clean install must call this first. The callback
+/// fires per chunk; callers should throttle.
+pub fn provision_onnx_runtime_with_progress(
+    local_dir: &Path,
+    mut on_progress: impl FnMut(&str, u64, Option<u64>),
+) -> Result<()> {
+    for (destination_file, source_url, archive_path) in ONNX_RUNTIME_FILES {
+        let destination = local_dir.join(destination_file);
+        if destination.exists() {
+            continue;
+        }
+        // Strip the "onnxruntime/" prefix for a clean label.
+        let label = destination_file
+            .rsplit('/')
+            .next()
+            .unwrap_or(destination_file);
+        download::download_nuget_file(
+            source_url,
+            archive_path,
+            &destination,
+            |downloaded, total| on_progress(label, downloaded, total),
+        )?;
+    }
+    Ok(())
+}
+
+/// Number of ONNX Runtime DLLs still missing from `local_dir` (for progress
+/// step counts before downloading).
+pub fn missing_onnx_runtime_count(local_dir: &Path) -> usize {
+    ONNX_RUNTIME_FILES
+        .iter()
+        .filter(|(destination_file, _, _)| !local_dir.join(destination_file).exists())
+        .count()
+}
+
+/// Download all missing runtime DLLs without progress reporting.
+pub fn download_onnx_runtime_files(local_dir: &Path) -> Result<()> {
+    if !cfg!(target_os = "windows") {
+        anyhow::bail!("ONNX Runtime DLL download is currently configured for Windows builds");
+    }
+
+    for (destination_file, source_url, archive_path) in ONNX_RUNTIME_FILES {
+        let destination = local_dir.join(destination_file);
+        download::download_nuget_file(source_url, archive_path, &destination, |_, _| {})?;
+    }
+
+    Ok(())
+}