fix: remove nested aggregate in claim_strength materialized view

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

node/spore_node/db/migrations/002_primitives.sql

@@ -78,10 +78,11 @@ SELECT
         NULLIF(COUNT(DISTINCT a.id)::float, 0),
         0
     ) AS net_sentiment,
-    MAX(GREATEST(
-        COALESCE(MAX(a.created_at), c.created_at),
-        COALESCE(MAX(e.created_at), c.created_at)
-    )) AS last_activity
+    GREATEST(
+        MAX(a.created_at),
+        MAX(e.created_at),
+        c.created_at
+    ) AS last_activity
 FROM claims c
 LEFT JOIN attestations a ON a.claim_id = c.id
 LEFT JOIN evidence e ON e.claim_id = c.id