Add missing InternService.new_show() method

Session.reset() calls intern_service.new_show() to reset per-show state,
but the method was never defined (added in 376265e without the impl).
Clears pending_interjection, pending_sources, and research_cache.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/services/intern.py

@@ -584,6 +584,11 @@ class InternService:
         self.pending_interjection = None
         self.pending_sources = []
 
+    def new_show(self):
+        self.pending_interjection = None
+        self.pending_sources = []
+        self.research_cache = {}
+
     @staticmethod
     def _clean_for_tts(text: str) -> str:
         if not text: