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/database/LocalStorage.py

@@ -1,7 +1,4 @@
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-  from server.GameBoard import GameBoard
+lazy from server.GameBoard import GameBoard
 
 from server.dataset.Dataset import Dataset
 from server.Files import save_file

server/database/backend/__init__.py

@@ -1,15 +1,17 @@
 from .Template import GameplayBackendTemplate
 
+lazy from .PostgresqlGameplayBackend import PostgresqlGameplayBackend as PostgresqlBackend
+lazy from .SqliteGameplayBackend import SqliteGameplayBackend as SqliteBackend
+
 class GameplayBackendBuilder:
   @staticmethod
   def build(backend:str="sqlite", db_path:str|None=None, busy_timeout_ms:int=5000, pg_dsn:str|None=None, pg_min_size:int=1, pg_max_size:int=5) -> GameplayBackendTemplate:
     normalized = (backend or "sqlite").strip().lower()
 
     if normalized == "postgresql" or normalized == "postgres":
-      from .PostgresqlGameplayBackend import PostgresqlGameplayBackend
       if not pg_dsn:
         raise ValueError("pg_dsn is required for the postgresql backend")
-      return PostgresqlGameplayBackend(
+      return PostgresqlBackend(
         dsn=pg_dsn,
         min_size=pg_min_size,
         max_size=pg_max_size,
@@ -17,9 +19,8 @@ class GameplayBackendBuilder:
       )
 
     if normalized == "sqlite":
-      from .SqliteGameplayBackend import SqliteGameplayBackend
       if not db_path:
         raise ValueError("db_path is required for the sqlite backend")
-      return SqliteGameplayBackend(db_path=db_path, busy_timeout_ms=busy_timeout_ms)
+      return SqliteBackend(db_path=db_path, busy_timeout_ms=busy_timeout_ms)
 
     raise ValueError(f"Unknown gameplay backend: {backend!r}. Choose 'sqlite' or 'postgresql'.")

server/database/benchmark_states.py

@@ -1,7 +1,4 @@
 """Load sampled gameplay positions from SQLite or PostgreSQL."""
-
-from __future__ import annotations
-
 import asyncio
 import json
 from urllib.parse import urlparse