Art: drag pieces into a new order

Each gallery tile gets a small drag handle (top-left, ✱). Grab it and
drop it anywhere among the others - the tile moves live as you drag
over neighbours, then the new order is saved automatically on drop.

- Artwork gets a `position` column; the gallery lists by position (was
  upload date). New uploads land after everything else.
- New POST /art/reorder endpoint takes an ordered list of ids (one
  shared CSRF token, "reorder-art") and rewrites positions 1..n.
- Migration backfills position from the existing upload-date order via
  ROW_NUMBER(), then drops the bootstrap default so it matches the
  Doctrine mapping.
- New Stimulus controller (art-reorder): native HTML5 drag/drop, no
  library. dragover live-reorders the DOM by comparing cursor x to the
  hovered tile's midpoint; dragend posts the resulting id order and
  shows a small "Saving order… / Order saved" status pill.

Verified: schema validate clean: a throwaway script exercised
persist+reload of a reordered set against the real DB, and a live
browser check dispatched real DragEvents through the actual Stimulus
controller and confirmed the exact POST payload it sent. phpunit 9/9.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

migrations/Version20260915151210.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260915151210 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add artwork.position for the drag-to-reorder gallery, backfilled by upload date';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE artwork ADD position INT NOT NULL DEFAULT 0');
+
+        // Backfill existing rows so the gallery keeps its current (upload-date) order
+        // until someone drags a piece; new uploads get their position in PHP.
+        $this->addSql(<<<'SQL'
+            UPDATE artwork a
+            JOIN (
+                SELECT id, ROW_NUMBER() OVER (ORDER BY uploaded_at ASC, id ASC) AS rn
+                FROM artwork
+            ) ranked ON ranked.id = a.id
+            SET a.position = ranked.rn
+            SQL);
+
+        // Drop the bootstrap default now that every row has a real position;
+        // the mapping has no default, so schema:validate expects none either.
+        $this->addSql('ALTER TABLE artwork CHANGE position position INT NOT NULL');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE artwork DROP position');
+    }
+}