feat: add upsert method to SupabaseClient for arena seekers

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

config/database.php

@@ -64,6 +64,21 @@ class SupabaseClient {
         return $this->request('DELETE', $url);
     }
 
+    public function upsert(string $table, array $data, string $onConflict = ''): array {
+        $url = $this->baseUrl . '/' . $table;
+        $headers = $this->headers();
+        $headers = array_map(function($h) use ($onConflict) {
+            if (str_starts_with($h, 'Prefer:')) {
+                return 'Prefer: return=representation,resolution=merge-duplicates';
+            }
+            return $h;
+        }, $headers);
+        if ($onConflict) {
+            $url .= '?on_conflict=' . $onConflict;
+        }
+        return $this->request('POST', $url, $data, $headers);
+    }
+
     public function rpc(string $fn, array $data = []): array {
         $url = $this->baseUrl . '/rpc/' . $fn;
         return $this->request('POST', $url, $data);