Fix devcontainer build, event-loop scope, and UTC timestamps

Found by running the stack for the first time:

- compose build context pointed outside the repo. Relative paths in
  .devcontainer/compose.override.yaml resolve against the project
  directory (the repo root), not the file's own directory, so "context: .."
  escaped the repository and the image could not build at all.
- The devcontainers/python base image ships a yarn apt source whose
  signing key has rotated, failing apt-get update and the whole build.
  Drop that source list; we don't use yarn.
- pytest-asyncio ran fixtures on a session-scoped loop while tests ran on
  per-function loops, so asyncmy raised "Future attached to a different
  loop" on every database-backed test. aiosqlite masked this; MySQL does
  not. Fixture loop scope now matches the test loop scope.
- MySQL DATETIME stores no offset, so timestamps serialized bare and left
  clients guessing. Connections are pinned to UTC, so DeviceRead now
  attaches that offset explicitly, with a test covering it.

Verified end to end against real MySQL 8.4 and Keycloak 26.7: cold start
from destroyed volumes, uv sync --frozen, migrations, 41 tests, ruff,
mypy --strict, and the live authorization matrix driven by real tokens.

Co-Authored-By: Claude Opus 5 <[email protected]>

pyproject.toml

@@ -81,7 +81,10 @@ ignore_missing_imports = true
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
-asyncio_default_fixture_loop_scope = "session"
+# Must match the test loop scope (function). If fixtures run on a session loop
+# while tests run on a per-function loop, asyncmy's loop-bound futures raise
+# "attached to a different loop". aiosqlite hides this; MySQL does not.
+asyncio_default_fixture_loop_scope = "function"
 testpaths = ["tests"]
 addopts = "-ra --strict-markers"
 filterwarnings = ["error"]