fix: clear all mypy errors

Annotate untyped containers and Optional attributes, type the stdlib-shaped
metric dicts in the client plugins, and add types-PyYAML/types-psutil to the
dev extras plus a mypy override for nio/mattermostdriver (no stubs).

Behaviour fixes found on the way: enable_kernel_timestamps() now returns False
when SO_TIMESTAMP is unknown instead of raising TypeError, send_shutdown is
defined at module level so cleanup() cannot hit a NameError, and a failed
subprocess returncode falls back to UNKNOWN.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

hbd/server/data.py

@@ -1,4 +1,4 @@
-msgs = []  # in-memory list of recent messages for new websocket clients; also logged to file via notify.eventlog
+msgs: list = []  # in-memory list of recent messages for new websocket clients; also logged to file via notify.eventlog
 
 
 class Data: