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.

tests/Unit/Entity/ContributionCollectionTest.php

@@ -0,0 +1,43 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Entity;
+
+use App\Entity\Contribution;
+use App\Entity\ContributionCollection;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+final class ContributionCollectionTest extends TestCase
+{
+    #[Test]
+    public function it_counts_zero_when_empty(): void
+    {
+        $collection = new ContributionCollection();
+
+        $this->assertCount(0, $collection);
+    }
+
+    #[Test]
+    public function it_counts_the_contributions_it_wraps(): void
+    {
+        $collection = new ContributionCollection(
+            new Contribution('github', 1_700_000_000, 1),
+            new Contribution('github', 1_700_086_400, 2),
+        );
+
+        $this->assertCount(2, $collection);
+    }
+
+    #[Test]
+    public function it_is_iterable_over_its_contributions(): void
+    {
+        $first  = new Contribution('github', 1_700_000_000, 1);
+        $second = new Contribution('gitlab', 1_700_086_400, 2);
+
+        $collection = new ContributionCollection($first, $second);
+
+        $this->assertSame([$first, $second], iterator_to_array($collection));
+    }
+}