feat(aggregator): fetch all providers concurrently

aggregate() now fires every configured provider's startFetch() in one
pass before resolving any of them, so wall-clock cost is roughly
max(providers) instead of sum(providers). Partial-failure isolation
and per-date summation behavior are unchanged.

src/Service/ContributionAggregator.php

@@ -19,7 +19,7 @@ final class ContributionAggregator
     /** @return array<string, int> */
     public function aggregate(): array
     {
-        $contributions = [];
+        $pending = [];
 
         /** @var ProviderInterface $provider */
         foreach ($this->providers as $provider) {
@@ -28,7 +28,17 @@ final class ContributionAggregator
             }
 
             try {
-                foreach ($provider->fetch() as $date => $count) {
+                $pending[] = [$provider, $provider->startFetch()];
+            } catch (\Throwable $e) {
+                $this->logger->warning(sprintf('%s fetch failed: %s', $provider::class, $e->getMessage()), ['exception' => $e]);
+            }
+        }
+
+        $contributions = [];
+
+        foreach ($pending as [$provider, $handle]) {
+            try {
+                foreach ($provider->resolveFetch($handle) as $date => $count) {
                     $contributions[$date] = ($contributions[$date] ?? 0) + $count;
                 }
             } catch (\Throwable $e) {