test: validate M3 imports and locking on PostgreSQL

Add isolated PostgreSQL fixtures, two-session locking coverage, and a disposable remote Docker test runner. Correct the canonical-root fixture and document verified SQLite and PostgreSQL results.

tests/Integration/RepositoryTest.php

@@ -27,7 +27,15 @@ final class RepositoryTest extends TestCase
     protected function tearDown(): void
     {
         unset($this->pdo);
-        unlink($this->database);
+        RepositoryDatabase::cleanup($this->database);
+    }
+
+    public function testSecondConnectionUsesTheMigratedDatabase(): void
+    {
+        $jobs = new PdoImportJobRepository($this->pdo);
+        $job = $jobs->create('/shared', []);
+        $other = new PdoImportJobRepository(RepositoryDatabase::connect($this->database));
+        self::assertSame('/shared', $other->get($job)['source_path']);
     }
 
     public function testCatalogGroupingAndFileLifecycle(): void
@@ -126,7 +134,7 @@ final class RepositoryTest extends TestCase
         $catalog = new PdoCatalogRepository($this->pdo);
         $job = $jobs->create('/', [['source_path' => '/a', 'size' => 5, 'model_id' => null]]);
         $second = $jobs->create('/', []);
-        $other = new PdoImportJobRepository(new PDO('sqlite:' . $this->database));
+        $other = new PdoImportJobRepository(RepositoryDatabase::connect($this->database));
         self::assertSame($job, (int) $jobs->claimNext()['id']);
         self::assertSame($second, (int) $other->claimNext()['id']);
         self::assertNull($other->claimNext());