fix(moderation): windowValue must not fall back to the 30d total on a null column

windowValue mapped only the 24h/7d keys and used `?? row.d30` as the fallback:

    const col = { '24h': row.d1, '7d': row.d7 }[key] ?? row.d30

so a null d1/d7 (which the function is documented to tolerate) returned the
30-day count instead of 0, inflating the 24h/7d moderation tiles. It happens to
be masked today because `SUM(created_at >= ?)` nulls d1/d7/d30 only in unison,
but the contract is wrong and the existing test used an all-null row that hid it.

Map all three window keys explicitly so each reads its own column and a null
coerces to 0 via `Number(col) || 0`. Add a regression test with a null narrow
column and a non-null d30.

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

server/src/model/moderation/moderation.pure.js

@@ -40,7 +40,7 @@ function reshapeWindows(rows) {
 // { d1, d7, d30 } sum row, coercing to a number and tolerating a null row.
 function windowValue(row, key) {
   if (!row) return 0
-  const col = { '24h': row.d1, '7d': row.d7 }[key] ?? row.d30
+  const col = { '24h': row.d1, '7d': row.d7, '30d': row.d30 }[key]
   return Number(col) || 0
 }