feat(growth): same-day 1-day baselines for new videos

Let 1-day view growth use a zero baseline for videos published today, and the first successful sample for older videos ingested today, without mixing those into 7/30-day rankings.

reaction_board/analytics.py

@@ -313,15 +313,20 @@ def compute_growth(
     current: int | float | None,
     previous: int | float | None,
     gap_days: int | float | None,
+    *,
+    allow_zero_gap: bool = False,
 ) -> Growth:
     """计算累计差值、日均差值和相对增速。
 
-    缺失基线或间隔不足 1 天时返回不可用，不把 0 当作真实增长。
+    缺失基线或间隔为负时返回不可用。默认间隔不足 1 天也不可用，不把 0 当成真实增长。
+    ``allow_zero_gap=True`` 时允许同一天的两次采样（或相对 0）计算增量，``per_day`` 为 None。
     """
     if current is None or previous is None or gap_days is None:
         return Growth(available=False)
     gap = int(gap_days)
-    if gap <= 0:
+    if gap < 0:
+        return Growth(available=False)
+    if gap == 0 and not allow_zero_gap:
         return Growth(available=False)
     current_value = max(0, int(current))
     previous_value = max(0, int(previous))
@@ -329,7 +334,7 @@ def compute_growth(
     return Growth(
         available=True,
         delta=delta,
-        per_day=delta / gap,
+        per_day=None if gap == 0 else delta / gap,
         rate=delta / max(previous_value, 1),
         gap_days=gap,
     )