refactor: add is_available method to fetchers

lrcfetch/fetchers/base.py

@@ -22,6 +22,11 @@ class BaseFetcher(ABC):
         """True if this fetcher manages its own cache (skip per-source cache check)."""
         return False
 
+    @abstractmethod
+    def is_available(self, track: TrackMeta) -> bool:
+        """Check if the fetcher is available for the given track (e.g. has required metadata)."""
+        pass
+
     @abstractmethod
     def fetch(
         self, track: TrackMeta, bypass_cache: bool = False