feat(audio): loudness_cache gains a content_key column (table only)

An ISRC or fingerprint column plus a partial index, with
set_content_key / get_by_content_key helpers, so a local or Plex copy of
the same recording can reuse an analysis. The analyser and every audio
path are untouched; set/get keep their track_id contract.

cc

crates/qbz-audio/src/loudness_cache.rs

@@ -47,6 +47,17 @@ impl LoudnessCache {
         )
         .map_err(|e| format!("Failed to create loudness table: {}", e))?;
 
+        // Identity beyond the Qobuz id (2026-08-28): an ISRC or a content
+        // fingerprint, so a local/Plex copy of the same recording can reuse
+        // the analysis. TABLE ONLY — the analyser and every audio path are
+        // untouched; `set`/`get` keep their track_id contract. Additive,
+        // idempotent (the duplicate-column error is the "already there" case).
+        let _ = conn.execute_batch("ALTER TABLE track_loudness ADD COLUMN content_key TEXT;");
+        let _ = conn.execute_batch(
+            "CREATE INDEX IF NOT EXISTS idx_track_loudness_content_key
+               ON track_loudness(content_key) WHERE content_key IS NOT NULL;",
+        );
+
         log::info!("[LoudnessCache] Opened at {}", db_path.display());
 
         Ok(Self {
@@ -71,6 +82,39 @@ impl LoudnessCache {
         .ok()
     }
 
+    /// Attach a cross-source content key (ISRC or fingerprint) to an
+    /// existing row. No-op when the track has no analysis yet.
+    pub fn set_content_key(&self, track_id: u64, content_key: &str) {
+        if content_key.trim().is_empty() {
+            return;
+        }
+        if let Ok(conn) = self.conn.lock() {
+            let _ = conn.execute(
+                "UPDATE track_loudness SET content_key = ?1 WHERE track_id = ?2",
+                params![content_key.trim(), track_id as i64],
+            );
+        }
+    }
+
+    /// Look up cached loudness by content key (any source's copy of the
+    /// same recording). Newest analysis wins.
+    pub fn get_by_content_key(&self, content_key: &str) -> Option<CachedLoudness> {
+        let conn = self.conn.lock().ok()?;
+        conn.query_row(
+            "SELECT gain_db, peak, source FROM track_loudness
+              WHERE content_key = ?1 ORDER BY created_at DESC LIMIT 1",
+            params![content_key.trim()],
+            |row| {
+                Ok(CachedLoudness {
+                    gain_db: row.get::<_, f64>(0)? as f32,
+                    peak: row.get::<_, f64>(1)? as f32,
+                    source: row.get(2)?,
+                })
+            },
+        )
+        .ok()
+    }
+
     /// Store or update loudness data for a track.
     pub fn set(&self, track_id: u64, gain_db: f32, peak: f32, source: &str) {
         if let Ok(conn) = self.conn.lock() {