feat!: adopt Python 3.15 and support BattlesnakeArena

- Require Python 3.15 in project metadata, runtime pin, and Docker stages.
- Use lazy imports for database backends and type-only dependencies.
- Defer benchmark dependencies until their code paths are used.
- Rely on deferred annotations and use Path.info for file discovery.
- Accept BattlesnakeArena alongside BattlesnakeEngine on game endpoints.
- Advance quart_common for Python 3.15 and multi-agent validation.
- Add pytest development dependencies and refresh the uv lockfile.
- Test lazy backend loading, import order, and fresh file discovery.
- Document the runtime requirements and locked uv workflow.

BREAKING CHANGE: Python 3.15 or newer is required; earlier runtimes cannot parse the lazy import syntax.

tests/test_DatabasePackageImports.py

@@ -4,6 +4,30 @@ import unittest
 from pathlib import Path
 
 class TestDatabasePackageImports(unittest.TestCase):
+  def test_builder_works_after_direct_backend_module_import(self):
+    from server.database.backend.SqliteGameplayBackend import SqliteGameplayBackend
+    from server.database.backend import GameplayBackendBuilder
+
+    backend = GameplayBackendBuilder.build(db_path=":memory:")
+    self.assertIsInstance(backend, SqliteGameplayBackend)
+
+  def test_builder_import_defers_backend_modules(self):
+    project_root = Path(__file__).resolve().parents[1]
+    script = """
+import sys
+from server.database.backend import GameplayBackendBuilder
+assert 'server.database.backend.SqliteGameplayBackend' not in sys.modules
+assert 'server.database.backend.PostgresqlGameplayBackend' not in sys.modules
+backend = GameplayBackendBuilder.build(db_path=':memory:')
+assert backend.__class__.__name__ == 'SqliteGameplayBackend'
+assert 'server.database.backend.PostgresqlGameplayBackend' not in sys.modules
+"""
+    result = subprocess.run(
+      [sys.executable, "-c", script], cwd=project_root,
+      text=True, capture_output=True,
+    )
+    self.assertEqual(result.returncode, 0, result.stderr)
+
   def test_sqlite_backend_import_does_not_require_aiofiles(self):
     project_root = Path(__file__).resolve().parents[1]
     script = """