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/threshold.py

@@ -53,16 +53,16 @@ class AlertState:
         self.metric_path = metric_path
         self.level = AlertLevel.OK
         self.since = time.time()
-        self.last_value = None
+        self.last_value: Any = None
         self.last_check = time.time()
         self.notification_count = 0
-        self.last_notification = None
-        self.threshold_value = None  # The threshold value that triggered alert
-        self.operator = None  # The comparison operator (>, <, >=, etc.)
+        self.last_notification: Optional[float] = None
+        self.threshold_value: Optional[float] = None  # The threshold value that triggered alert
+        self.operator: Optional[str] = None  # The comparison operator (>, <, >=, etc.)
         self.hysteresis: Optional[float] = None  # Hysteresis fraction used for recovery
-        self.formatted_message = None  # Formatted display message for UI
+        self.formatted_message: Optional[str] = None  # Formatted display message for UI
         self.acknowledged = False  # Whether alert has been acknowledged
-        self.acknowledged_at = None  # Timestamp when acknowledged
+        self.acknowledged_at: Optional[float] = None  # Timestamp when acknowledged
         self.consecutive_count = 0  # Consecutive exceedances while still OK (for count gating)
         self.pending_since: Optional[float] = None  # non-None while waiting out grace period before notifying
 
@@ -362,13 +362,13 @@ class ThresholdChecker:
             journal: Optional MessageJournal instance for logging threshold events
         """
         # Named threshold configurations (pre-merged: defaults + overrides): {config_name: {metric_path: ThresholdConfig}}
-        self.threshold_configs = {}
+        self.threshold_configs: Dict[str, Dict[str, ThresholdConfig]] = {}
 
         # Raw overrides only for each named config (no defaults baked in): {config_name: {metric_path: ThresholdConfig}}
         self.threshold_raw_configs: Dict[str, Dict[str, ThresholdConfig]] = {}
 
         # Single threshold set for backward compatibility: {metric_path: ThresholdConfig}
-        self.thresholds = {}
+        self.thresholds: Dict[str, ThresholdConfig] = {}
 
         # Host to ordered list of config names: {host_name: [config_name, ...]}
         self.host_config_mapping: Dict[str, List[str]] = {}
@@ -1276,7 +1276,7 @@ class ThresholdChecker:
 
     def _format_display(
         self,
-        display_format: str,
+        display_format: Optional[str],
         value: Any,
         threshold_value: Optional[float],
         op_symbol: str,