feat(snmp): NAS presets for Synology and Asustor, reading every disk and volume (#5985, #5986)

A preset can now read a vendor's tables: one row per disk, volume, fan or
sensor. The scheduled poll discovers the rows by walking the device: on its
first poll, after a save, and then hourly, and only while it answers. Each row
becomes readings named by the device's own labels ("Disk 1 status"). Only the
columns a row answered are asked for.

A vendor status, whether a code (Synology) or a word (Asustor), is a state
reading. It is stored as a level (0 OK, 1 busy, 2 warning, 3 failed), so one
alert rule covers any vendor, and shown in the device's own word.

- Synology: system, power and fan status and temperature; per disk: status,
  health, temperature and bad sectors; per volume: RAID state (scrubbing,
  parity check, rebuild, degraded), free space and size.
- Asustor: per disk: status, SMART and temperature; per volume: status, free
  space and size; CPU and system temperature, free memory and fans.
  From ADM's ASUSTOR-DISK-MIB and ASUSTOR-SYSTEM-MIB.
- Linux server: plus each physical disk's 1-minute busy % (UCD-DISKIO-MIB).

Migration snmp_002_table_rows adds tables, tables_at and texts to
snmp_devices. Milestone 530.

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

plugins/snmp/service.py

@@ -11,7 +11,7 @@ from steward.core.crypto import decrypt_secret, encrypt_secret
 
 from .models import SnmpDevice
 from .poller import AUTH_PROTOCOLS, PRIV_PROTOCOLS, Credentials
-from .readings import PRESETS, Reading, device_readings, parse_interfaces
+from .readings import PRESETS, Reading, device_readings, parse_interfaces, tables_for
 
 VERSIONS = ("1", "2c", "3")
 PRESET_IDS = set(PRESETS) | {"interfaces"}
@@ -22,13 +22,15 @@ class FormError(ValueError):
     """What is wrong with a submitted device form, said to the person."""
 
 
-def readings_for(device: SnmpDevice) -> list[Reading]:
-    """What the device is asked for. A row saved through the form always
-    parses; one that somehow doesn't is asked for nothing rather than raising
-    in the poll loop."""
+def readings_for(device: SnmpDevice, tables: dict | None = None) -> list[Reading]:
+    """What the device is asked for, its vendor tables' rows included (the
+    stored ones, or `tables` when the poll has just found them). A row saved
+    through the form always parses; one that somehow doesn't is asked for
+    nothing rather than raising in the poll loop."""
     try:
         return device_readings(device.presets or [], parse_interfaces(device.interfaces),
-                               device.custom, device.version)
+                               device.custom, device.version,
+                               device.tables if tables is None else tables)
     except ValueError:
         return []
 
@@ -84,7 +86,9 @@ def parse_form(form, existing: SnmpDevice | None = None) -> dict:
         numbers = parse_interfaces(interfaces)
         if "interfaces" in presets and not numbers:
             raise FormError("Interface traffic needs the interface numbers to read, e.g. 1, 2 or 1-4.")
-        if not device_readings(presets, numbers, custom, version):
+        # A NAS preset's rows are found by the first poll, so it reads
+        # nothing until then and still counts as a choice.
+        if not device_readings(presets, numbers, custom, version) and not tables_for(presets):
             raise FormError("Choose at least one preset or add a custom reading.")
     except ValueError as exc:
         raise FormError(str(exc)) from None