feat(store): finish ContributionStore and add typed entities

- Fix add()'s upsert SQL (was invalid SQL from typos) and use it for
  the new merge() as well.
- Fix remove()'s missing FROM keyword and drop the unindexed LIKE scan
  in favor of an exact match.
- Add latestDate(), all() with optional sinceDays filtering, and
  prune() with a retention-day cutoff.
- Add Contribution (immutable value object) and ContributionCollection
  (IteratorAggregate + Countable) so all() returns something typed
  instead of a raw date => count array.
- Cover all of the above with unit tests against an in-memory SQLite
  database.

src/Entity/Contribution.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Entity;
+
+final class Contribution
+{
+    public function __construct(
+        public readonly string $provider,
+        public readonly int $date,
+        public readonly int $count,
+    ) {
+    }
+}