Persist endpoint disk health

Adds diskhealthjson with its migration in the same change, so the model never references a column the schema has not applied.

Kept separate from disksjson rather than merged into it. That column is per volume and answers how full a drive is; this one is per physical device and answers whether the hardware is dying. They come from different WMI sources with different failure modes, so merging them would make a capacity collection failure indistinguishable from a health collection failure.

Nullable with no backfill: a drive whose health has never been reported is not a healthy drive, and defaulting to an empty list would read as no disks have problems on every endpoint predating the column.

plugins/endpoints/models/endpoint.py

@@ -102,6 +102,12 @@ class EndpointState(db.Model):
     # is deliberately distinct from a confident False.
     pendingreboot = db.Column(db.Boolean, nullable=True)
 
+    # Per-drive health from the agent's two disk sources. JSON because a machine
+    # has a variable number of drives and each carries its own verdict. Kept
+    # separate from disksjson: that is capacity, this is whether the hardware is
+    # dying. Null = never reported.
+    diskhealthjson = db.Column(db.Text(length=16777215), nullable=True)
+
     # Warn-only reconciler output, refreshed on every agent check-in in the same
     # commit as servicesjson. Each entry names a policy/shipped-attached monitor
     # the endpoint is expected to be running but the latest snapshot does not