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.

server/blueprints/metrics.py

@@ -1,8 +1,6 @@
 from quart import Blueprint, jsonify, request
-from typing import TYPE_CHECKING
 
-if TYPE_CHECKING:
-  from server.Server import Server
+lazy from server.Server import Server
 
 def create_metrics_blueprint(server:'Server') -> Blueprint:
   blueprint = Blueprint('metrics', __name__)