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]>

.devcontainer/Dockerfile

@@ -1,7 +1,12 @@
 FROM mcr.microsoft.com/devcontainers/python:1-3.13-bookworm
 
 # `mysql` CLI for poking at the database during development.
-RUN apt-get update \
+#
+# The base image ships a yarn apt source whose signing key has since rotated,
+# which makes `apt-get update` fail outright. We don't use yarn, so drop that
+# source list rather than carrying its key.
+RUN rm -f /etc/apt/sources.list.d/yarn.list \
+    && apt-get update \
     && apt-get install -y --no-install-recommends default-mysql-client \
     && rm -rf /var/lib/apt/lists/*