Fix: "Clear my history" now clears account history too

clearSession only reset the device-local history; for a signed-in user the panel
shows the account history (serverHistory), which was never cleared and never sent
to the server — so it looked like nothing happened. Add DELETE /api/history
(clear all) and have clearSession reset serverHistory + call it when signed in.

goodnews/api.py

@@ -563,6 +563,14 @@ def create_app() -> FastAPI:
             conn.commit()
         return {"ok": True}
 
+    @app.delete("/api/history")
+    def clear_history(request: Request) -> dict:
+        with get_conn() as conn:
+            user = _require_user(conn, request)
+            conn.execute("DELETE FROM user_history WHERE user_id = ?", (user["id"],))
+            conn.commit()
+        return {"ok": True}
+
     @app.delete("/api/history/{article_id}")
     def remove_history(article_id: int, request: Request) -> dict:
         with get_conn() as conn: