feat(comments) : add file attachments on comments

Comments can now have documents attached via multipart/form-data upload.
New endpoint GET /api/documents/comment/{id} to list a comment's files.
Document entity gains a comment relation with cascade remove.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/Version20260323160000.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260323160000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add comment_id FK on documents table';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql("DO \$\$ BEGIN IF NOT EXISTS (SELECT 1 FROM information_schema.columns WHERE table_name = 'documents' AND column_name = 'comment_id') THEN ALTER TABLE documents ADD COLUMN comment_id VARCHAR(36) DEFAULT NULL; END IF; END \$\$");
+        $this->addSql("DO \$\$ BEGIN IF NOT EXISTS (SELECT 1 FROM pg_constraint WHERE conname = 'fk_documents_comment') THEN ALTER TABLE documents ADD CONSTRAINT fk_documents_comment FOREIGN KEY (comment_id) REFERENCES comments(id) ON DELETE CASCADE; END IF; END \$\$");
+        $this->addSql('CREATE INDEX IF NOT EXISTS idx_documents_comment_id ON documents(comment_id)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE documents DROP CONSTRAINT IF EXISTS fk_documents_comment');
+        $this->addSql('DROP INDEX IF EXISTS idx_documents_comment_id');
+        $this->addSql('ALTER TABLE documents DROP COLUMN IF EXISTS comment_id');
+    }
+}