cleanup: remove dead code and duplicate docs

- Remove session-ses_2f27.md (161KB raw session log)
- Remove 49 ROOT_* duplicate files across REFERENCE/
- Remove 14 duplicate files between REFERENCE/ root and history/
- Remove asr_legacy.rs (dead code, replaced by asr.rs)
- Remove src/core/worker/ (duplicate JobWorker)
- Remove src/core/layers/ (empty directory)
- Remove 4 .bak files in src/
- Remove 7 dead private methods in worker/processor.rs
- Remove backup directory from git tracking

src/core/probe/ffprobe.rs

@@ -20,6 +20,8 @@ pub struct StreamInfo {
     pub duration: Option<String>,
     pub sample_rate: Option<String>,
     pub channels: Option<u32>,
+    #[serde(default)]
+    pub tags: Option<serde_json::Value>,
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -69,6 +71,7 @@ pub fn probe_video(video_path: &str) -> Result<ProbeResult> {
                     duration: s["duration"].as_str().map(String::from),
                     sample_rate: s["sample_rate"].as_str().map(String::from),
                     channels: s["channels"].as_u64().map(|v| v as u32),
+                    tags: s.get("tags").cloned(),
                 })
                 .collect()
         })