fix: preserve gameplay data and correct duel evaluation

- Preserve snake customizations across database migrations and merges.
- Lazily load optional storage backends for SQLite maintenance scripts.
- Match Apex territory and nearest-food tie-breaking semantics.
- Resolve duel occupancy after simultaneous movement and food growth.
- Recompute simulated head-to-head danger after body growth.
- Add regression coverage and declare the aiofiles dependency.

tests/test_DatabasePackageImports.py

@@ -0,0 +1,32 @@
+import subprocess
+import sys
+import unittest
+from pathlib import Path
+
+class TestDatabasePackageImports(unittest.TestCase):
+  def test_sqlite_backend_import_does_not_require_aiofiles(self):
+    project_root = Path(__file__).resolve().parents[1]
+    script = """
+import builtins
+
+original_import = builtins.__import__
+def reject_aiofiles(name, *args, **kwargs):
+  if name == 'aiofiles' or name.startswith('aiofiles.'):
+    raise ModuleNotFoundError("aiofiles intentionally unavailable")
+  return original_import(name, *args, **kwargs)
+
+builtins.__import__ = reject_aiofiles
+from server.database.backend.SqliteGameplayBackend import SqliteGameplayBackend
+assert SqliteGameplayBackend.__name__ == 'SqliteGameplayBackend'
+"""
+    result = subprocess.run(
+      [sys.executable, "-c", script],
+      cwd=project_root,
+      text=True,
+      capture_output=True,
+    )
+
+    self.assertEqual(result.returncode, 0, result.stderr)
+
+if __name__ == "__main__":
+  unittest.main()