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/battlesnake.py

@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, cast
+from typing import cast
 import asyncio, json, time, os
 
 from quart import Blueprint, request, jsonify
@@ -10,8 +10,7 @@ from snakes import DEFAULT_SNAKE_CONFIG
 from server.GameBoard import GameBoard
 from server.Files import read_file
 
-if TYPE_CHECKING:
-  from server.Server import Server
+lazy from server.Server import Server
 
 def create_battlesnake_blueprint(server:'Server') -> Blueprint:
   blueprint = Blueprint('battlesnake', __name__)
@@ -45,7 +44,7 @@ def create_battlesnake_blueprint(server:'Server') -> Blueprint:
     return snake_json
 
   @blueprint.post('/start')
-  @require_user_agent("BattlesnakeEngine", abort_code=404)
+  @require_user_agent(("BattlesnakeEngine", "BattlesnakeArena"), abort_code=404)
   async def on_start():
     server.metrics_collector.record_http_request('start')
     await server.game_runtime.prune_stale_games()
@@ -59,7 +58,7 @@ def create_battlesnake_blueprint(server:'Server') -> Blueprint:
     return 'ok'
 
   @blueprint.post('/move')
-  @require_user_agent("BattlesnakeEngine", abort_code=404)
+  @require_user_agent(("BattlesnakeEngine", "BattlesnakeArena"), abort_code=404)
   async def on_move():
     server.metrics_collector.record_http_request('move')
     game_state = await request.get_json()
@@ -91,7 +90,7 @@ def create_battlesnake_blueprint(server:'Server') -> Blueprint:
     return {'move': next_move}
 
   @blueprint.post('/end')
-  @require_user_agent("BattlesnakeEngine", abort_code=404)
+  @require_user_agent(("BattlesnakeEngine", "BattlesnakeArena"), abort_code=404)
   async def on_end():
     server.metrics_collector.record_http_request('end')
     await server.game_runtime.prune_stale_games()