docs(phpdoc): document public methods across src/

Add professional PHPDoc (summary + @param/@return) to the remaining
public methods that had none or only partial coverage: GraphController's
actions, ContributionStore's accessors, SvgRenderer's render() and
private helpers (with concrete array-shape annotations), ProbeTrait,
ProviderHealthChecker, ProviderStatus, and the Contribution entity.
Also adds the matching @return shapes on SvgRendererTest's data
providers.

src/Service/Provider/ProviderHealthChecker.php

@@ -8,13 +8,17 @@ use Symfony\Component\DependencyInjection\Attribute\AutowireIterator;
 
 final class ProviderHealthChecker
 {
+    /** @param iterable<ProviderInterface> $providers */
     public function __construct(
         #[AutowireIterator('app.provider')]
         private readonly iterable $providers,
     ) {}
 
     /**
-     * @return array{status: string, providers: array<string, array<string, string>>}
+     * Probes every configured provider and aggregates the results for the /health endpoint.
+     *
+     * @return array{status: string, providers: array<string, array<string, string>>} 'status' is
+     *   "degraded" if any provider's probe() reported an error, otherwise "ok"
      */
     public function check(): array
     {