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/Service/ContributionStoreTest.php

@@ -0,0 +1,113 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Unit\Service;
+
+use App\Service\ContributionStore;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+final class ContributionStoreTest extends TestCase
+{
+    #[Test]
+    public function it_stores_and_retrieves_a_contribution(): void
+    {
+        $store = new ContributionStore(':memory:');
+
+        $store->add('github', 1_700_000_000, 4);
+        $all = $store->all('github');
+
+        $this->assertCount(1, $all);
+        $this->assertSame(4, iterator_to_array($all)[0]->count);
+    }
+
+    #[Test]
+    public function it_upserts_on_a_repeated_date(): void
+    {
+        $store = new ContributionStore(':memory:');
+
+        $store->add('github', 1_700_000_000, 4);
+        $store->add('github', 1_700_000_000, 9);
+        $all = $store->all('github');
+
+        $this->assertCount(1, $all);
+        $this->assertSame(9, iterator_to_array($all)[0]->count);
+    }
+
+    #[Test]
+    public function it_removes_a_contribution(): void
+    {
+        $store = new ContributionStore(':memory:');
+
+        $store->add('github', 1_700_000_000, 4);
+        $store->remove('github', 1_700_000_000);
+
+        $this->assertCount(0, $store->all('github'));
+    }
+
+    #[Test]
+    public function it_merges_a_batch_of_date_counts(): void
+    {
+        $store = new ContributionStore(':memory:');
+
+        $store->merge('github', [1_700_000_000 => 1, 1_700_086_400 => 2]);
+
+        $this->assertCount(2, $store->all('github'));
+    }
+
+    #[Test]
+    public function it_reports_the_latest_date_for_a_provider(): void
+    {
+        $store = new ContributionStore(':memory:');
+
+        $store->add('github', 1_700_000_000, 1);
+        $store->add('github', 1_700_086_400, 2);
+
+        $this->assertSame(1_700_086_400, $store->latestDate('github'));
+    }
+
+    #[Test]
+    public function it_returns_null_latest_date_when_provider_has_no_rows(): void
+    {
+        $store = new ContributionStore(':memory:');
+
+        $this->assertNull($store->latestDate('github'));
+    }
+
+    #[Test]
+    public function it_filters_all_by_since_days(): void
+    {
+        $store = new ContributionStore(':memory:');
+        $now   = time();
+
+        $store->add('github', $now - 10 * 86400, 1);
+        $store->add('github', $now - 400 * 86400, 2);
+
+        $this->assertCount(1, $store->all('github', sinceDays: 30));
+    }
+
+    #[Test]
+    public function it_does_not_prune_when_retention_is_unset(): void
+    {
+        $store = new ContributionStore(':memory:', retentionDays: null);
+
+        $store->add('github', time() - 1_000 * 86400, 1);
+        $store->prune();
+
+        $this->assertCount(1, $store->all('github'));
+    }
+
+    #[Test]
+    public function it_prunes_rows_older_than_the_retention_window(): void
+    {
+        $store = new ContributionStore(':memory:', retentionDays: 30);
+        $now   = time();
+
+        $store->add('github', $now - 10 * 86400, 1);
+        $store->add('github', $now - 40 * 86400, 2);
+        $store->prune();
+
+        $this->assertCount(1, $store->all('github'));
+    }
+}