fix(dupont): resolve YYYY-Qn / YYYY-Hn period windows (500 on quarter/half)

Root cause: resolve_window() matched quarter/half with p[4] == "q" / "h"
(always '-' for "2026-q2"/"2026-h1"), and the branches sat after the
single-month branch -> dead code. Real input fell into the month branch,
so months_between() raised ValueError on int('q2') -> HTTP 500 for all
18 quarter/half options in the period selector.

Fix: judge on p[5] in ("q","h") with a digit guard on p[6], evaluated
before the single-month branch; month branch tightened to p[5:].isdigit().
Existing error-branch wording unchanged.

Tests: backend/tests/test_dupont_period_options.py (18 defect options,
case equivalence, month/year/ytd/ttm/custom non-regression, malformed
input returns error dict instead of raising).

backend/app/utils/dupont_basis.py

@@ -197,17 +197,17 @@ def resolve_window(period: Optional[str], start: Optional[str], end: Optional[st
             return {"error": "自定义区间须同时给出 start 与 end（YYYY-MM）"}
         s, e = start, end
         key = "custom"
-    elif len(p) == 7 and p[4] == "-":            # 2026-08 → 单月
-        s = e = p
-        key = "month"
-    elif len(p) == 7 and p[4] == "q":            # 2026-q3
+    elif len(p) == 7 and p[5] == "q" and p[6].isdigit():   # 2026-q3
         y, q = int(p[:4]), int(p[6])
         s, e = f"{y}-{(q - 1) * 3 + 1:02d}", f"{y}-{q * 3:02d}"
         key = "quarter"
-    elif len(p) == 7 and p[4] == "h":            # 2026-h1
+    elif len(p) == 7 and p[5] == "h" and p[6].isdigit():   # 2026-h1
         y, h = int(p[:4]), int(p[6])
         s, e = f"{y}-{(h - 1) * 6 + 1:02d}", f"{y}-{h * 6:02d}"
         key = "half"
+    elif len(p) == 7 and p[5:].isdigit():        # 2026-08 → 单月
+        s = e = p
+        key = "month"
     elif len(p) == 4 and p.isdigit():            # 2026 → 全年
         s, e = f"{p}-01", f"{p}-12"
         key = "year"

backend/tests/test_dupont_period_options.py

@@ -0,0 +1,121 @@
+# -*- coding: utf-8 -*-
+"""杜邦区间选择器 —— 季度(YYYY-Qn)/半年(YYYY-Hn) 解析回归用例。
+
+缺陷背景（project-20261004-ask08-dupont-quarter-half-500-fix）：
+  `resolve_window()` 的季度/半年分支判据写成 `p[4] == "q" / "h"`（应为 `p[5]`），
+  且排在单月分支（`p[4] == "-"`）之后 ⇒ 死代码；真实入参 `2026-q2` / `2026-h1`
+  （端点内 `.lower()`）落进单月分支 → `months_between()` 的 `int(end[5:7])`
+  抛 `ValueError: invalid literal for int() ... 'q2'` → HTTP 500。
+
+本用例只测纯函数 `resolve_window`（不连库、不写盘、不碰服务），
+口径与前端 `frontend/src/views/DupontAnalysis.vue::periodOptions` 对齐。
+"""
+import pytest
+
+from app.utils.dupont_basis import resolve_window
+
+YEARS = (2024, 2025, 2026)
+AVAIL = [f"{y}-{m:02d}" for y in YEARS for m in range(1, 13)]
+
+
+def rw(period, **kw):
+    kw.setdefault("today", "2026-10")
+    return resolve_window(period, None, None, AVAIL, **kw)
+
+
+# ── 正例：季度 / 半年（缺陷面 18 项） ───────────────────────────────────────
+
+@pytest.mark.parametrize("year,q,s,e", [
+    (2026, 1, "2026-01", "2026-03"),
+    (2026, 2, "2026-04", "2026-06"),
+    (2026, 3, "2026-07", "2026-09"),
+    (2026, 4, "2026-10", "2026-12"),
+    (2025, 2, "2025-04", "2025-06"),
+    (2024, 4, "2024-10", "2024-12"),
+])
+def test_quarter_parses(year, q, s, e):
+    w = rw(f"{year}-Q{q}")
+    assert "error" not in w
+    assert w["period_key"] == "quarter"
+    assert (w["period_start"], w["period_end"]) == (s, e)
+    assert w["months"] == 3
+
+
+@pytest.mark.parametrize("year,h,s,e", [
+    (2026, 1, "2026-01", "2026-06"),
+    (2026, 2, "2026-07", "2026-12"),
+    (2025, 1, "2025-01", "2025-06"),
+    (2024, 2, "2024-07", "2024-12"),
+])
+def test_half_parses(year, h, s, e):
+    w = rw(f"{year}-H{h}")
+    assert "error" not in w
+    assert w["period_key"] == "half"
+    assert (w["period_start"], w["period_end"]) == (s, e)
+    assert w["months"] == 6
+
+
+def test_all_18_defect_options_parse():
+    """前端选择器同一区间内的 3 年 × (2 半年 + 4 季度) = 18 项全部可解析。"""
+    bad = []
+    for y in YEARS:
+        for h in (1, 2):
+            w = rw(f"{y}-H{h}")
+            if w.get("period_key") != "half" or w.get("months") != 6:
+                bad.append(f"{y}-H{h} -> {w}")
+        for q in (1, 2, 3, 4):
+            w = rw(f"{y}-Q{q}")
+            if w.get("period_key") != "quarter" or w.get("months") != 3:
+                bad.append(f"{y}-Q{q} -> {w}")
+    assert bad == [], bad
+
+
+def test_lower_and_upper_case_equivalent():
+    """端点内 `.lower()` 后入参为小写；大小写必须同解且区间一致。"""
+    assert rw("2026-Q2") == rw("2026-q2")
+    assert rw("2026-H1") == rw("2026-h1")
+
+
+# ── 反例：单月 / 全年 / YTD / TTM / 自定义 口径不得被季度半年分支改动 ──────
+
+@pytest.mark.parametrize("month", ["2026-01", "2026-08", "2026-12"])
+def test_single_month_not_swallowed_by_quarter_half(month):
+    w = rw(month)
+    assert "error" not in w
+    assert w["period_key"] == "month", w
+    assert w["period_start"] == w["period_end"] == month
+    assert w["months"] == 1
+
+
+def test_full_year_still_year():
+    w = rw("2026")
+    assert w["period_key"] == "year"
+    assert (w["period_start"], w["period_end"]) == ("2026-01", "2026-12")
+    assert w["months"] == 12
+
+
+def test_ytd_ttm_unchanged():
+    ytd = rw("ytd", anchor="2026-10")
+    assert ytd["period_key"] == "ytd"
+    assert (ytd["period_start"], ytd["period_end"]) == ("2026-01", "2026-10")
+    assert ytd["months"] == 10
+
+    ttm = rw("ttm", anchor="2026-10")
+    assert ttm["period_key"] == "ttm"
+    assert (ttm["period_start"], ttm["period_end"]) == ("2025-10", "2026-10")
+    assert ttm["months"] == 13
+
+
+def test_custom_unchanged():
+    w = resolve_window("custom", "2026-01", "2026-06", AVAIL, today="2026-10")
+    assert w["period_key"] == "custom"
+    assert (w["period_start"], w["period_end"]) == ("2026-01", "2026-06")
+    assert w["months"] == 6
+
+
+@pytest.mark.parametrize("junk", ["2026-qx", "2026-hx", "2026-x3", "q2", "2026-1"])
+def test_malformed_period_returns_error_dict_not_raise(junk):
+    """无法识别的区间参数必须回落 error dict（端点转 4xx），绝不抛异常。"""
+    w = rw(junk)
+    assert isinstance(w, dict)
+    assert w.get("error"), (junk, w)