Fix --since in welcomebot-reputation

A relative bound has to be datetime('now', '-24 hours'); datetime('-24 hours')
returns NULL, so the comparison was false for every row and --since silently
reported nothing rather than erroring. Caught when a --since '-10 minutes'
came back empty for queries that had demonstrably just happened.

Absolute forms ('2026-09-13') still take the single-argument path.

bin/welcomebot-reputation

@@ -92,6 +92,18 @@ def limited(sql):
     return sql if limit == "all" else f"{sql} LIMIT {int(limit)}"
 
 
+def since_expr(column):
+    """SQL for the --since bound.
+
+    A leading +/- means "relative to now" — sqlite needs that spelled
+    datetime('now', '-24 hours'), NOT datetime('-24 hours'), which returns
+    NULL and silently matches no rows.
+    """
+    if since.startswith(("+", "-")):
+        return f"{column} >= datetime('now', ?)"
+    return f"{column} >= datetime(?)"
+
+
 def tables_exist():
     names = {r[0] for r in conn.execute(
         "SELECT name FROM sqlite_master WHERE type = 'table'")}
@@ -106,7 +118,7 @@ if not tables_exist():
 if mode == "stats":
     where, params = [], []
     if since:
-        where.append("queried_at >= datetime(?)")
+        where.append(since_expr("queried_at"))
         params.append(since)
     clause = (" WHERE " + " AND ".join(where)) if where else ""
     row = conn.execute(
@@ -150,7 +162,7 @@ if mode == "blocks":
         where.append("kind = ?")
         params.append(kind)
     if since:
-        where.append("first_blocked_at >= datetime(?)")
+        where.append(since_expr("first_blocked_at"))
         params.append(since)
     clause = (" WHERE " + " AND ".join(where)) if where else ""
     rows = conn.execute(limited(
@@ -179,7 +191,7 @@ if kind:
     where.append("kind = ?")
     params.append(kind)
 if since:
-    where.append("queried_at >= datetime(?)")
+    where.append(since_expr("queried_at"))
     params.append(since)
 clause = (" WHERE " + " AND ".join(where)) if where else ""