Fix artist-detail showing wrong artist when a source id is duplicated

A pasted Deezer artist link (or any Deezer-source artist click) opened the
wrong artist's header: deezer_id 525046 is stamped on 4 library rows (Kendrick
+ 3 others — an enrichment-corruption bug), and the library-upgrade lookup did
WHERE deezer_id=? LIMIT 1, grabbing an arbitrary row (Jorja Smith) while the
discography loaded fresh from Deezer (Kendrick) — a Frankenstein page.

find_library_artist_for_source now detects when a source id maps to >1 library
artist and refuses to guess: it skips the id-based upgrade (still allowing the
name fallback), so the caller renders the source artist directly — landing on
the correct artist. Unique ids are unaffected (no regression).

The underlying enrichment bug that writes one source id onto multiple artists
is separate and still worth a follow-up.

core/artist_source_lookup.py

@@ -53,7 +53,11 @@ def find_library_artist_for_source(
 
     Lookup order:
       1. Direct match on the source-specific ID column (server-agnostic — any
-         library record with the right external ID is a hit).
+         library record with the right external ID is a hit). If that id is
+         stamped on MORE than one library artist, the mapping is corrupt /
+         ambiguous (e.g. an enrichment bug wrote one Deezer id onto several
+         artists) — we refuse to guess and fall through, so the caller can
+         show the source artist directly instead of an arbitrary wrong one.
       2. Case-insensitive name match within ``active_server`` (defaults to the
          active media server when not provided), so we don't jump the user
          across server contexts on a name collision.
@@ -67,13 +71,23 @@ def find_library_artist_for_source(
     try:
         with database._get_connection() as conn:
             cursor = conn.cursor()
+            # LIMIT 2 so we can tell a unique match from an ambiguous one.
             cursor.execute(
-                f"SELECT id, name FROM artists WHERE {column} = ? LIMIT 1",
+                f"SELECT id FROM artists WHERE {column} = ? LIMIT 2",
                 (str(source_artist_id),),
             )
-            row = cursor.fetchone()
-            if row:
-                return row[0]
+            rows = cursor.fetchall()
+            if len(rows) == 1:
+                return rows[0][0]
+            if len(rows) > 1:
+                # Same source id on multiple artists — corrupt mapping. Don't
+                # upgrade on the id; fall through to the name match (and, if
+                # that misses, let the caller render the source artist).
+                logger.warning(
+                    f"Source id {source}:{source_artist_id} maps to "
+                    f"{len(rows)}+ library artists — ambiguous, skipping "
+                    f"id-based library upgrade"
+                )
 
             if artist_name and active_server:
                 cursor.execute(

tests/metadata/test_artist_source_lookup.py

@@ -164,6 +164,45 @@ class TestFindLibraryArtistForSource:
         )
         assert result is None
 
+    def test_ambiguous_source_id_skips_id_upgrade(self, db):
+        """Regression for the Kendrick/Jorja bug: when one Deezer id is
+        stamped on several library artists (enrichment corruption), the id
+        match is ambiguous and must NOT pick an arbitrary row — it returns
+        None so the caller falls back to showing the source artist."""
+        _insert_artist(db, artist_id="pk-kendrick", name="Kendrick Lamar",
+                       deezer_id="525046", server_source="plex")
+        _insert_artist(db, artist_id="pk-jorja", name="Jorja Smith",
+                       deezer_id="525046", server_source="plex")
+        _insert_artist(db, artist_id="pk-vince", name="Vince Staples",
+                       deezer_id="525046", server_source="plex")
+
+        # No name hint (the URL-driven path) → no id guess, no name fallback.
+        assert find_library_artist_for_source(
+            db, "deezer", "525046", active_server="plex"
+        ) is None
+
+    def test_ambiguous_source_id_still_allows_name_fallback(self, db):
+        """An ambiguous id shouldn't block a correct name match when the
+        caller does have the name."""
+        _insert_artist(db, artist_id="pk-kendrick", name="Kendrick Lamar",
+                       deezer_id="525046", server_source="plex")
+        _insert_artist(db, artist_id="pk-jorja", name="Jorja Smith",
+                       deezer_id="525046", server_source="plex")
+
+        result = find_library_artist_for_source(
+            db, "deezer", "525046", artist_name="Kendrick Lamar",
+            active_server="plex",
+        )
+        assert result == "pk-kendrick"
+
+    def test_unique_source_id_still_matches(self, db):
+        """Positive control: a non-duplicated id still upgrades as before."""
+        _insert_artist(db, artist_id="pk-solo", name="Solo Artist",
+                       deezer_id="999999", server_source="plex")
+        assert find_library_artist_for_source(
+            db, "deezer", "999999"
+        ) == "pk-solo"
+
     def test_id_match_wins_over_name_match(self, db):
         """If both a source-id match and a name match exist, the id match
         should take priority — it's the more reliable signal."""