feat: add GET /shows/by-episode/{episode_number} endpoint

Allows looking up shows by episode number instead of internal DB ID,
enabling IRC bot commands like !playlist 530 to resolve directly.

Made-with: Cursor

src/ntr_fetcher/api.py

@@ -89,6 +89,20 @@ def create_app(
             for s in shows
         ]
 
+    @app.get("/shows/by-episode/{episode_number}")
+    def show_by_episode(episode_number: int):
+        show = db.get_show_by_episode_number(episode_number)
+        if show is None:
+            raise HTTPException(status_code=404, detail=f"No show with episode number {episode_number}")
+        tracks = db.get_show_tracks(show.id)
+        return {
+            "show_id": show.id,
+            "episode_number": show.episode_number,
+            "week_start": show.week_start.isoformat(),
+            "week_end": show.week_end.isoformat(),
+            "tracks": tracks,
+        }
+
     @app.get("/shows/{show_id}")
     def show_detail(show_id: int):
         shows = db.list_shows(limit=1000, offset=0)