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/oauth.py

@@ -128,7 +128,7 @@ def get_providers(config: dict) -> list[ResolvedProvider]:
     Entries with missing required fields or unknown types are skipped with
     a warning log.  Order follows config declaration order.
     """
-    result = []
+    result: list = []
     oauth_cfg = config.get("oauth", {})
     if not isinstance(oauth_cfg, dict):
         return result