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.

scripts/benchmark_snakes_from_db.py

@@ -1,8 +1,5 @@
 #!/usr/bin/env python3
 """Benchmark snake move latency against sampled states from gameplay SQLite."""
-
-from __future__ import annotations
-
 import argparse
 from pathlib import Path
 import sqlite3
@@ -15,8 +12,8 @@ sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
 from server.database.benchmark_states import (
   _build_state, is_postgresql_source, load_postgresql_states,
 )
-from server.GameBoard import GameBoard
-from snakes import SnakeBuilder
+lazy from server.GameBoard import GameBoard
+lazy from snakes import SnakeBuilder
 
 def percentile(values: list[float], quantile: float) -> float:
   ordered = sorted(values)