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/migrations/versions/0014_endpoint_diskhealth.py

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+"""Add diskhealthjson to endpointstates.
+
+Per-drive health, kept separate from disksjson. That column answers "how full
+is this drive"; this one answers "is this drive dying". Merging them would mean
+a capacity collection failure and a health collection failure could not be told
+apart, and the two come from different WMI sources with different failure modes.
+
+JSON rather than scalar columns because a machine has a variable number of
+drives and each carries its own verdict, and because the agent reports the same
+drive from two sources - MSFT_PhysicalDisk for NVMe and Win32_DiskDrive for
+ATA/SATA - which the server keeps side by side rather than collapsing.
+
+MEDIUMTEXT for consistency with the other snapshot columns on this table.
+
+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 that predates this column.
+
+Idempotent: the column-add checks the live schema first, so a re-run and a
+db.create_all() site both stay clean on strict MySQL 8.
+"""
+from alembic import op
+
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'endpoints0014diskhealth'
+down_revision = 'endpoints0013inventorycolumns'
+branch_labels = None
+depends_on = None
+
+TABLE_NAME = 'endpointstates'
+COLUMN_NAME = 'diskhealthjson'
+
+
+def _has_column(table_name, column_name):
+    from sqlalchemy import inspect
+
+    inspector = inspect(op.get_bind())
+    if not inspector.has_table(table_name):
+        return False
+    return any(col['name'] == column_name
+               for col in inspector.get_columns(table_name))
+
+
+def upgrade():
+    if not _has_column(TABLE_NAME, COLUMN_NAME):
+        op.add_column(
+            TABLE_NAME,
+            sa.Column(COLUMN_NAME, sa.Text(length=16777215), nullable=True))
+
+
+def downgrade():
+    if _has_column(TABLE_NAME, COLUMN_NAME):
+        op.drop_column(TABLE_NAME, COLUMN_NAME)