feat: QR PNG 생성 + vote URL resolver

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

tests/e2e.py

@@ -299,6 +299,38 @@ def t_topics_seeded_after_assign():
         assert c["tone"]
 
 
+def t_make_qr_png():
+    from app import make_qr_png
+    png = make_qr_png("http://localhost:8501/?mode=vote")
+    # PNG signature 8 bytes
+    assert png[:8] == b"\x89PNG\r\n\x1a\n"
+    assert len(png) > 100
+
+
+def t_compute_vote_url_priority():
+    import os as _os
+    from app import compute_vote_url, save_data, load_data, _empty_state
+    save_data(_empty_state())
+
+    # 1. settings.public_base_url 우선
+    d = load_data()
+    d["settings"]["public_base_url"] = "http://example.com:9000"
+    save_data(d)
+    assert compute_vote_url() == "http://example.com:9000/?mode=vote"
+
+    # 2. env fallback
+    d["settings"].pop("public_base_url")
+    save_data(d)
+    _os.environ["PUBLIC_BASE_URL"] = "http://env-host:7777"
+    assert compute_vote_url() == "http://env-host:7777/?mode=vote"
+    _os.environ.pop("PUBLIC_BASE_URL")
+
+    # 3. localhost / LAN fallback
+    url = compute_vote_url()
+    assert url.endswith("/?mode=vote")
+    assert url.startswith("http://")
+
+
 if __name__ == "__main__":
     print(f"# E2E (data={TEST_DATA})\n")
     test("hackathon.json 로드 (34명, 7팀)", t_load)
@@ -318,6 +350,8 @@ if __name__ == "__main__":
     test("stage 헬퍼", t_stage_helpers)
     test("topics 헬퍼", t_topics_helpers)
     test("topics 시드", t_topics_seeded_after_assign)
+    test("QR PNG 생성", t_make_qr_png)
+    test("vote URL 우선순위", t_compute_vote_url_priority)
 
     fails = sum(1 for r, _ in results if r == FAIL)
     print(f"\n# {len(results)} 중 통과 {len(results) - fails}, 실패 {fails}")