fix(web): implement delete_cached so the font catalog cache actually invalidates

api_v3.py's font upload/delete handlers import delete_cached from
web_interface.cache, but the function was never defined. The surrounding
except ImportError silently swallowed the failure, so the fonts_catalog
cache entry survived uploads/deletes and newly uploaded fonts did not
appear until the TTL expired or the service restarted.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

web_interface/cache.py

@@ -29,6 +29,12 @@ def set_cached(key: str, value: Any, ttl_seconds: int = 60) -> None:
     _cache_timestamps[key] = time.time()
 
 
+def delete_cached(key: str) -> None:
+    """Remove a single key from the cache if present."""
+    _cache.pop(key, None)
+    _cache_timestamps.pop(key, None)
+
+
 def invalidate_cache(pattern: Optional[str] = None) -> None:
     """Invalidate cache entries matching pattern, or all if pattern is None."""
     if pattern is None: