fix(growth): rank the rate board by a smoothed score

Show the real percentage on the card, but sort with delta / (baseline + prior) so a small baseline cannot dominate. Same-day publishes stay off the rate board.

reaction_board/config.py

@@ -52,6 +52,7 @@ DEFAULT_ANALYTICS: dict[str, Any] = {
     "video_shrink_minutes": 60,
     "shrink_minutes": None,
     "home_growth_days": "auto",
+    "growth_rate_prior": 500,
 }
 
 DEFAULT_CONFIG: dict[str, Any] = {
@@ -210,6 +211,7 @@ class AnalyticsConfig:
     video_shrink_minutes: int
     shrink_minutes: int | None
     home_growth_days: str | int
+    growth_rate_prior: int
 
 
 @dataclass(frozen=True)
@@ -592,6 +594,11 @@ def _build_config(data: Mapping[str, Any], path: Path, *, secrets_path: Path) ->
             )
         ),
         home_growth_days=home_growth_days,
+        growth_rate_prior=_as_int(
+            analytics_raw.get("growth_rate_prior", 500),
+            "analytics.growth_rate_prior",
+            minimum=0,
+        ),
     )
 
     return AppConfig(