Fix phantom duplicate tracks created on Merged file ingestion

When the mover returns MoveOutcome::Merged (destination already exists,
source deleted), approve_and_finalize was checking only file_hash to
detect duplicates. Since the second ingestion had a different hash
(different quality/mastering), it bypassed the check and created a
phantom track record pointing to an existing storage_path with the
wrong hash (of the now-deleted source file).

Added a second dedup check by storage_path: if a non-hidden track
already exists at that path, mark pending as 'merged' instead of
inserting a new track row. This prevents phantom entries for any
subsequent ingestion of a different-quality version of an already
stored file.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

furumi-agent/src/db.rs

@@ -334,18 +334,31 @@ pub async fn approve_and_finalize(
         .fetch_one(pool)
         .await?;
 
-    // Check if track already exists (e.g. previously approved but pending not cleaned up)
+    // Check if track already exists by file_hash (re-approval of same file)
     let existing: Option<(i64,)> = sqlx::query_as("SELECT id FROM tracks WHERE file_hash = $1")
         .bind(&pt.file_hash)
         .fetch_optional(pool)
         .await?;
 
     if let Some((track_id,)) = existing {
-        // Already finalized — just mark pending as approved
         update_pending_status(pool, pending_id, "approved", None).await?;
         return Ok(track_id);
     }
 
+    // Check if track already exists by storage_path (Merged: different quality file landed
+    // at the same destination, source was deleted — don't create a phantom duplicate)
+    let existing_path: Option<(i64,)> = sqlx::query_as(
+        "SELECT id FROM tracks WHERE storage_path = $1 AND NOT hidden"
+    )
+    .bind(storage_path)
+    .fetch_optional(pool)
+    .await?;
+
+    if let Some((track_id,)) = existing_path {
+        update_pending_status(pool, pending_id, "merged", None).await?;
+        return Ok(track_id);
+    }
+
     let artist_name = pt.norm_artist.as_deref().unwrap_or("Unknown Artist");
     let artist_id = upsert_artist(pool, artist_name).await?;