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/models.py

@@ -16,6 +16,9 @@ class SnmpDevice(Base):
     Secrets (the community, the v3 keys) are stored encrypted. The last_*
     columns are the newest poll's outcome, written by the poller, so a page
     can say why a device isn't answering without asking the device itself.
+    `tables` is the rows the poller found in the presets' vendor tables (a
+    disk or volume each), as of `tables_at`; `texts` is each state reading's
+    newest answer in the device's own word.
     """
     __tablename__ = "snmp_devices"
 
@@ -40,3 +43,6 @@ class SnmpDevice(Base):
     last_ok_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     last_error: Mapped[str | None] = mapped_column(Text, nullable=True)
     missing: Mapped[list] = mapped_column(JSON, nullable=False, default=list)
+    tables: Mapped[dict] = mapped_column(JSON, nullable=False, default=dict)
+    tables_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    texts: Mapped[dict] = mapped_column(JSON, nullable=False, default=dict)