nodes: store and show host disk free; warn when low (node-26#41)

Checkin now carries disk_free_bytes/disk_total_bytes. Stored on the node doc
(not overwritten by older nodes that don't send them). Node page shows a Disk
row; node cards warn amber under 15% free, red under 5% or 2 GB. A full disk
stopped node-002 recording for 17 days while it showed Online.

Frontend unverified locally (no Node, #207); CI build typechecks.

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

drb-c2-core/app/internal/mqtt_handler.py

@@ -128,6 +128,8 @@ class MQTTHandler:
                 "sdr_devices": payload.get("sdr_devices"),
                 "op25_sdr_serial": payload.get("op25_sdr_serial"),
                 "sdr_count": payload.get("sdr_count"),  # None until reported, never a guessed 1
+                "disk_free_bytes": payload.get("disk_free_bytes"),
+                "disk_total_bytes": payload.get("disk_total_bytes"),
                 "enforce_override_timeout": payload.get("enforce_override_timeout", True),
                 "is_overridden": False,
                 "override_system_id": None,
@@ -161,8 +163,11 @@ class MQTTHandler:
             # radio_channels (node-26#24): a conventional node's channels and
             # whether each is encrypted — the one decode failure the owner has
             # to hear about from C2, since nothing on the node can fix it.
+            # disk_* (node-26#41): free/total bytes of the host disk the node
+            # records to — a full disk silently stopped node-002 for 17 days.
             for key in ("secondary_sdr_mode", "secondary_sdr_priority", "secondary_sdr_running",
-                        "sdr_pins", "sdr_devices", "op25_sdr_serial", "radio_channels"):
+                        "sdr_pins", "sdr_devices", "op25_sdr_serial", "radio_channels",
+                        "disk_free_bytes", "disk_total_bytes"):
                 if key in payload:
                     updates[key] = payload[key]
             if "sdr_count" in payload: