feat(tagger): add JoyTag provider behind a tagger-model abstraction

Introduces a selectable tagging model so Phokus isn't locked to the
anime-leaning WD tagger. JoyTag uses the Danbooru schema but generalizes to
photographic content and is strong on NSFW concepts — a better fit for a photo
manager while keeping the explicitness range.

- `TaggerModel` enum (wd | joytag) persisted as a `tagger_model` setting;
  `model_dir`, status, and download now follow the active model (per-model
  repo/dir/file list). WD stays the default.
- `Tagger` trait implemented by `WdTagger` and the new `JoyTagger`;
  `create_active_tagger` builds the selected one. The worker holds
  `Box<dyn Tagger>` and rebuilds on model change (TAGGER_SESSION_DIRTY).
- Shared `assemble_batch` skeleton (pack -> one forward pass -> per-image
  fallback, results in input order); both providers and the shared
  decode/pad/resize are de-duped onto common helpers.
- JoyTag specifics: NCHW + RGB + CLIP-normalized input (vs WD's NHWC/BGR/raw),
  flat top_tags.txt labels, logits -> sigmoid -> threshold (default 0.4). It has
  no native rating, so the explicitness rating is derived from its NSFW tags.
- Tags are attributed to the model that produced them (ai_tagger_model),
  via Tagger::model_name, instead of a hardcoded WD constant.
- New get/set_tagger_model commands.

Backend only; the Settings model picker and end-to-end testing against the
JoyTag model files come next.

src-tauri/src/commands.rs

@@ -8,7 +8,7 @@ use crate::db::{
 use crate::embedder;
 use crate::hnsw_index;
 use crate::indexer::{self, WatcherHandle};
-use crate::tagger::{self, TaggerAcceleration, TaggerModelStatus, TaggerRuntimeProbe};
+use crate::tagger::{self, TaggerAcceleration, TaggerModel, TaggerModelStatus, TaggerRuntimeProbe};
 use crate::vector;
 use serde::{Deserialize, Serialize};
 use std::collections::HashSet;
@@ -1970,6 +1970,11 @@ pub struct SetTaggerAccelerationParams {
     pub acceleration: TaggerAcceleration,
 }
 
+#[derive(Deserialize)]
+pub struct SetTaggerModelParams {
+    pub model: TaggerModel,
+}
+
 #[derive(Deserialize)]
 pub struct SetTaggerThresholdParams {
     pub threshold: f32,
@@ -2030,6 +2035,21 @@ pub async fn set_tagger_acceleration(
     tagger::set_tagger_acceleration(&app_dir, params.acceleration).map_err(|e| e.to_string())
 }
 
+#[tauri::command]
+pub async fn get_tagger_model(app: AppHandle) -> Result<TaggerModel, String> {
+    let app_dir = app.path().app_data_dir().map_err(|e| e.to_string())?;
+    Ok(tagger::tagger_model(&app_dir))
+}
+
+#[tauri::command]
+pub async fn set_tagger_model(
+    app: AppHandle,
+    params: SetTaggerModelParams,
+) -> Result<TaggerModel, String> {
+    let app_dir = app.path().app_data_dir().map_err(|e| e.to_string())?;
+    tagger::set_tagger_model(&app_dir, params.model).map_err(|e| e.to_string())
+}
+
 #[tauri::command]
 pub async fn probe_tagger_runtime(app: AppHandle) -> Result<TaggerRuntimeProbe, String> {
     let app_dir = app.path().app_data_dir().map_err(|e| e.to_string())?;