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/compose.override.yaml

@@ -1,16 +1,20 @@
 # Adds the development container to the stack defined in ../compose.yaml.
 # Merged by devcontainer.json; not useful on its own.
+#
+# NOTE: relative paths below resolve against the *project directory* -- the
+# directory of the first compose file (the repo root), not this file's own
+# directory. Hence "." rather than "..".
 
 services:
   app:
     build:
-      context: ..
+      context: .
       dockerfile: .devcontainer/Dockerfile
     command: sleep infinity
     ports:
       - "8000:8000"
     volumes:
-      - ..:/workspaces/v2x-server:cached
+      - .:/workspaces/v2x-server:cached
       # Keep the Linux venv out of the macOS/Windows bind mount. Without this,
       # host-side tooling and platform-specific wheels collide.
       - venv:/workspaces/v2x-server/.venv