Fix AcoustID test connection falsely failing for valid API keys

The fallback test (used when no audio files exist in the library) sends
a dummy fingerprint to the AcoustID API. The API correctly rejects the
dummy fingerprint but this is not error code 4 (invalid key), so the
test was returning False instead of True. Any non-code-4 error from the
fallback means the API key was accepted — only code 4 means a bad key.

core/acoustid_client.py

@@ -329,7 +329,9 @@ class AcoustIDClient:
                 # Error code 4 is specifically "invalid API key"
                 if error_code == 4:
                     return False, "Invalid AcoustID API key - get one from https://acoustid.org/new-application"
-                return False, f"AcoustID API error: {error_msg}"
+                # Any other error (e.g. "invalid fingerprint") means the API key
+                # was accepted — the dummy test fingerprint is just rejected as expected
+                return True, "AcoustID API key is valid"
 
             # Status is 'ok' - key is valid
             return True, "AcoustID API key is valid"

web_server.py

@@ -21601,6 +21601,7 @@ def get_version_info():
                     "• Docker compose includes optional slskd service block",
                     "• Multi-stage Docker build reduces image size",
                     "• MusicBrainz recording ID backfilled from Navidrome during scan",
+                    "• AcoustID test connection no longer reports failure for valid API keys when library is empty",
                 ],
             },
             # v2.2 and earlier features moved to archive