fix(api): move FILTER onto the aggregate in /api/stats

PostgreSQL only accepts FILTER on an aggregate call. It was attached to
ROUND, so the whole query was rejected and /api/stats answered 500. The
showcase reads that endpoint for its live creature stats, which meant the
homepage displayed five dashes instead of numbers.

FILTER now sits on AVG, with the rounding applied to the result.

api/server.js

@@ -336,9 +336,12 @@ app.get('/api/stats', async (req, res) => {
       SELECT
         COUNT(*) FILTER (WHERE is_alive = true) as alive_count,
         COUNT(*) FILTER (WHERE is_alive = false) as dead_count,
-        ROUND(AVG(hunger)::numeric, 1) FILTER (WHERE is_alive = true) as avg_hunger,
-        ROUND(AVG(happiness)::numeric, 1) FILTER (WHERE is_alive = true) as avg_happiness,
-        ROUND(AVG(energy)::numeric, 1) FILTER (WHERE is_alive = true) as avg_energy,
+        -- FILTER ne s'applique qu'a un agregat : accroche a ROUND, qui n'en
+        -- est pas un, PostgreSQL rejette toute la requete. Il doit porter sur
+        -- AVG, et l'arrondi vient ensuite.
+        ROUND(AVG(hunger) FILTER (WHERE is_alive = true)::numeric, 1) as avg_hunger,
+        ROUND(AVG(happiness) FILTER (WHERE is_alive = true)::numeric, 1) as avg_happiness,
+        ROUND(AVG(energy) FILTER (WHERE is_alive = true)::numeric, 1) as avg_energy,
         COUNT(*) FILTER (WHERE hunger > 80 AND is_alive = true) as starving_count,
         COUNT(*) FILTER (WHERE happiness < 20 AND is_alive = true) as sad_count
       FROM creatures