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.

pyproject.toml

@@ -7,7 +7,7 @@ name = "snake-python"
 version = "0.1.0"
 description = "Add your description here"
 readme = "README.md"
-requires-python = ">=3.13"
+requires-python = ">=3.15"
 dependencies = [
     "aiofiles>=25.1.0",
     "aiologger>=0.7.0",
@@ -18,3 +18,8 @@ dependencies = [
     "python-dotenv>=1.2.2",
     "asyncpg>=0.31.0",
 ]
+
+[dependency-groups]
+dev = [
+    "pytest>=9.2.0",
+]