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>

tests/integration/test_snmp_poll.py

@@ -85,3 +85,52 @@ def test_a_poll_records_why_a_device_is_silent_and_what_another_answered(app):
     # 60,000 bytes over the 1.1 s or more between the polls, stored as bits
     # per second: at most 436 kbit/s, and above 150 unless a poll took seconds.
     assert 150_000 < second["if1_in"] <= 436_364
+
+
+def test_a_nas_is_walked_once_then_read_by_row_its_states_stored_as_levels_and_shown_in_its_words(app):
+    from sqlalchemy import select
+
+    from steward.models.metrics import PluginMetricLatest
+
+    scheduler = _snmp("scheduler")
+    SnmpDevice = _snmp("models").SnmpDevice
+    poller = _snmp("poller")
+
+    name = f"nas-{uuid.uuid4().hex[:8]}"
+    walks = []
+
+    async def fake_walk(address, port, creds, tables):
+        walks.append([t.id for t in tables])
+        return poller.TableScan(rows={
+            "asustor_disks": [{"index": "1", "label": "Disk1", "columns": ["status", "temp"]}],
+            "asustor_volumes": [{"index": "1", "label": "Volume 1", "columns": ["status", "free"]}]})
+
+    async def fake_poll(address, port, creds, oids):
+        keys = {k for _, k in oids}
+        assert {"disk1_status", "disk1_temp", "volume1_status", "volume1_free"} <= keys
+        return poller.PollResult(readings={"disk1_temp": 41.0, "volume1_free": 100.0},
+                                 texts={"disk1_status": "Normal", "volume1_status": "Degraded"})
+
+    async def go():
+        from steward.core.crypto import encrypt_secret
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                s.add(SnmpDevice(name=name, address="10.9.9.5", presets=["asustor"],
+                                 community=encrypt_secret("public")))
+        scheduler.RATES = None
+        await scheduler._do_poll(app, poll=fake_poll, walk=fake_walk)
+        await scheduler._do_poll(app, poll=fake_poll, walk=fake_walk)
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                row = (await s.execute(select(SnmpDevice).where(SnmpDevice.name == name))).scalar_one()
+                latest = {r.metric_name: r.value for r in (await s.execute(select(PluginMetricLatest).where(
+                    PluginMetricLatest.source_module == "snmp", PluginMetricLatest.resource_name == name))).scalars()}
+                await s.delete(row)
+        return row, latest
+
+    row, latest = asyncio.run(go())
+    assert len(walks) == 1 and walks[0][:2] == ["asustor_disks", "asustor_volumes"]   # not again within the hour
+    assert row.tables_at is not None and row.tables["asustor_volumes"][0]["label"] == "Volume 1"
+    assert row.texts == {"disk1_status": "Normal", "volume1_status": "Degraded"} and row.last_error is None
+    assert latest["disk1_status"] == 0.0 and latest["volume1_status"] == 2.0 and latest["disk1_temp"] == 41.0
+    assert latest["volume1_free"] == 100.0 * 2 ** 30                   # ADM's GB, stored in bytes