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/indexer.rs

@@ -3,7 +3,7 @@ use crate::db::{self, DbPool, EmbeddingJob, FolderJobProgress, ImageRecord, Inde
 use crate::embedder::{embedding_source_path, ClipImageEmbedder};
 use crate::media::{probe_video_metadata, MediaTools};
 use crate::storage::{detect_storage_profile, RuntimeAdaptiveProfile, StorageProfile};
-use crate::tagger::{self, WdTagger};
+use crate::tagger::{self, Tagger};
 use crate::thumbnail;
 use crate::vector;
 use anyhow::Result;
@@ -330,7 +330,7 @@ pub fn start_caption_worker(app: AppHandle, pool: DbPool, app_data_dir: PathBuf)
 
 pub fn start_tagging_worker(app: AppHandle, pool: DbPool, app_data_dir: PathBuf) {
     std::thread::spawn(move || {
-        let mut tagger_instance: Option<WdTagger> = None;
+        let mut tagger_instance: Option<Box<dyn Tagger>> = None;
         log::info!("Tagging worker started.");
         loop {
             // If the acceleration setting changed, drop the cached session so
@@ -1236,7 +1236,7 @@ fn process_tagging_batch(
     app: &AppHandle,
     pool: &DbPool,
     app_data_dir: &Path,
-    tagger_instance: &mut Option<WdTagger>,
+    tagger_instance: &mut Option<Box<dyn Tagger>>,
 ) -> Result<bool> {
     if !tagger::tagger_model_status(app_data_dir).ready {
         return Ok(false);
@@ -1264,7 +1264,7 @@ fn process_tagging_batch(
     }
 
     if tagger_instance.is_none() {
-        match WdTagger::new(app_data_dir) {
+        match tagger::create_active_tagger(app_data_dir) {
             Ok(model) => *tagger_instance = Some(model),
             Err(error) => {
                 with_db_write_lock(|| {
@@ -1323,6 +1323,10 @@ fn process_tagging_batch(
     }
     let infer_elapsed = infer_started_at.elapsed();
 
+    // Attribute the tags to the model that actually produced them, not a
+    // hardcoded one (WD vs JoyTag are both possible).
+    let tagger_model_name = tagger_ref.model_name();
+
     let tag_results = jobs.iter().cloned().zip(outputs).collect::<Vec<_>>();
 
     let write_started_at = Instant::now();
@@ -1355,7 +1359,7 @@ fn process_tagging_batch(
                         job.image_id,
                         &tag_pairs,
                         &output.rating,
-                        tagger::WD_TAGGER_MODEL_NAME,
+                        tagger_model_name,
                     )?;
                 }
                 Err(error) => {