feat: stage 헬퍼 (get_stage, set_stage, can_accept_votes)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/e2e.py

@@ -225,6 +225,39 @@ def t_load_data_backfills_nested_settings():
     assert fresh["topics"] == {"categories": []}
 
 
+def t_stage_helpers():
+    from app import get_stage, set_stage, can_accept_votes, load_data, save_data, _empty_state
+    save_data(_empty_state())
+
+    assert get_stage() == "intro"
+    assert can_accept_votes(load_data()) is False  # intro → False
+
+    set_stage("vote")
+    d = load_data()
+    assert d["settings"]["current_stage"] == "vote"
+    assert d["settings"]["voting_open"] is True
+    assert can_accept_votes(d) is True
+
+    set_stage("topics")
+    d = load_data()
+    assert d["settings"]["current_stage"] == "topics"
+    assert d["settings"]["voting_open"] is True  # 떠나도 voting_open 유지
+    assert can_accept_votes(d) is False  # stage 다르면 False
+
+    # 명시적 마감 후 vote 다시 들어오면 voting_open 자동 True
+    d["settings"]["voting_open"] = False
+    save_data(d)
+    set_stage("vote")
+    assert can_accept_votes(load_data()) is True
+
+    # invalid stage → ValueError
+    try:
+        set_stage("invalid_stage")
+        raise AssertionError("expected ValueError")
+    except ValueError:
+        pass
+
+
 if __name__ == "__main__":
     print(f"# E2E (data={TEST_DATA})\n")
     test("hackathon.json 로드 (34명, 7팀)", t_load)
@@ -241,6 +274,7 @@ if __name__ == "__main__":
     test("clear_votes", t_vote_count)
     test("empty_state 신규 키", t_empty_state_has_topics_and_stage)
     test("load_data nested 키 backfill", t_load_data_backfills_nested_settings)
+    test("stage 헬퍼", t_stage_helpers)
 
     fails = sum(1 for r, _ in results if r == FAIL)
     print(f"\n# {len(results)} 중 통과 {len(results) - fails}, 실패 {fails}")