fix: atomic import swap, disk/DB replace consistency, guarded pipeline loop

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/lyra_worker/pipeline.py

@@ -80,6 +80,16 @@ def _already_in_library_at_cutoff(conn: psycopg.Connection, target: MBTarget, cu
         return cur.fetchone() is not None
 
 
+def _library_quality(conn: psycopg.Connection, target: MBTarget) -> int | None:
+    with conn.cursor() as cur:
+        cur.execute(
+            'SELECT "qualityClass" FROM "LibraryItem" WHERE artist = %s AND album = %s',
+            (target.artist, target.album),
+        )
+        row = cur.fetchone()
+        return row[0] if row else None
+
+
 def _persist_candidates(conn: psycopg.Connection, job_id: str, candidates: list[Candidate]) -> None:
     with conn.cursor() as cur:
         for c in candidates:
@@ -211,16 +221,24 @@ def run_pipeline(
             return
 
         final = album_dir(dest_root, target)
-        import_album(staging, final)  # clean audio(+cover) move into the library, replacing any prior copy
-
-        if tagger is not None:
-            try:
-                tagger.tag_album(final, target)
-            except Exception as e:  # a tagging failure must not discard a good download
-                print(f"pipeline: tagging failed for job {job_id}: {e}", flush=True)
-
-        # 6. import (DB)
+        existing_q = _library_quality(conn, target)
+        new_q = quality_class(winner.quality)
         _set_state(conn, job_id, "imported", "import")
-        _import(conn, job_id, target, winner, final)
+        if existing_q is None or new_q > existing_q:
+            import_album(staging, final)  # clean audio(+cover) move, atomically replacing any prior copy
+            if tagger is not None:
+                try:
+                    tagger.tag_album(final, target)
+                except Exception as e:  # a tagging failure must not discard a good download
+                    print(f"pipeline: tagging failed for job {job_id}: {e}", flush=True)
+            _import(conn, job_id, target, winner, final)
+        else:
+            # an existing copy is already at >= this quality — keep it untouched, just complete the request
+            with conn.cursor() as cur:
+                cur.execute(
+                    "UPDATE \"Request\" SET status = 'completed' WHERE id = %s",
+                    (_request_id(conn, job_id),),
+                )
+            conn.commit()
     finally:
         shutil.rmtree(staging, ignore_errors=True)  # a partial/failed download never persists