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/ContributionCollection.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Entity;
+
+/**
+ * @implements \IteratorAggregate<int, Contribution>
+ */
+final class ContributionCollection implements \IteratorAggregate, \Countable
+{
+    /** @var array<int, Contribution> */
+    private readonly array $contributions;
+
+    public function __construct(Contribution ...$contributions)
+    {
+        $this->contributions = $contributions;
+    }
+
+    public function getIterator(): \ArrayIterator
+    {
+        return new \ArrayIterator($this->contributions);
+    }
+
+    public function count(): int
+    {
+        return count($this->contributions);
+    }
+}