feat(snmp): devices you add, test and read on the SNMP page (milestone 491)

The operator, reviewing SNMP on #5214: "this system looks incomplete like
there's something missing from it". The one device shown was plugin.yaml's
example (core-switch 192.168.1.1, community public), polled every minute;
devices could only be defined by editing YAML in the image; a failed poll was
logged at DEBUG and stored nowhere, so the page said "waiting for the next
poll" forever; v3 ran as v2c; counters showed as lifetime totals.

- Devices are rows (snmp_devices, migration snmp_001): address, port,
  v1/v2c community or a v3 user with auth/priv protocols and keys (secrets
  encrypted), presets plus custom readings, an optional host binding, and
  the newest poll's outcome. The YAML device list and its example are gone.
- The SNMP page adds, edits and deletes them (admin, audited, with
  get_audit_entities so the audit log names and links a device). Readings
  come from presets (system uptime, interface traffic by number, UPS-MIB,
  Linux net-snmp) and custom "OID name [unit]" lines.
- One GET per device with every OID, devices polled at once. The outcome is
  written to the row in words ("No reply from … within 5 s. Check the address
  and port, that SNMP is on, and the community"), with readings it lacks.
  v1's whole-request error drops that OID and asks again. SNMPv3 signs in
  with USM (MD5/SHA-1/SHA-2, DES/3DES/AES).
- Test asks the unsaved form's device for its name, description, uptime and
  interfaces by number. It is an admin's explicit action; every page reads
  only stored rows (System 178's ruling).
- Each reading has a unit; a counter is stored as a per-second rate (bytes as
  bit/s), a reset or wrap skipped. The page, widget and host panel say values
  in units and why a device is silent; the device page draws a chart per
  reading on a dated axis.

Tests: readings and presets, the form and its secrets, v3 checks, rates,
the poller against a stand-in pysnmp (one GET, v1 retry, timeout wording,
USM), the pages, and an integration test of the poll writing outcomes and
rates on Postgres.

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

plugins/snmp/migrations/versions/snmp_001_devices.py

@@ -0,0 +1,49 @@
+"""SNMP devices are rows edited on the SNMP page
+
+Devices lived in plugin.yaml's config, which the UI could not edit, and a poll
+that failed left no trace. A row per device holds what to ask and how, and the
+newest poll's outcome (milestone 491).
+
+Revision ID: snmp_001_devices
+Revises: (none — a branch off core via depends_on)
+Create Date: 2026-10-08
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "snmp_001_devices"
+down_revision: Union[str, None] = None
+branch_labels: Union[str, Sequence[str], None] = "snmp"
+depends_on: Union[str, Sequence[str], None] = ("0004_app_settings",)
+
+
+def upgrade() -> None:
+    op.create_table(
+        "snmp_devices",
+        sa.Column("id", sa.String(36), primary_key=True),
+        sa.Column("name", sa.String(100), nullable=False, unique=True),
+        sa.Column("address", sa.String(255), nullable=False),
+        sa.Column("port", sa.Integer, nullable=False, server_default="161"),
+        sa.Column("version", sa.String(3), nullable=False, server_default="2c"),
+        sa.Column("community", sa.Text, nullable=False, server_default=""),
+        sa.Column("v3_user", sa.String(100), nullable=False, server_default=""),
+        sa.Column("v3_auth", sa.String(16), nullable=False, server_default="none"),
+        sa.Column("v3_auth_key", sa.Text, nullable=False, server_default=""),
+        sa.Column("v3_priv", sa.String(16), nullable=False, server_default="none"),
+        sa.Column("v3_priv_key", sa.Text, nullable=False, server_default=""),
+        sa.Column("presets", sa.JSON, nullable=False, server_default="[]"),
+        sa.Column("interfaces", sa.String(255), nullable=False, server_default=""),
+        sa.Column("custom", sa.Text, nullable=False, server_default=""),
+        sa.Column("host_id", sa.String(36), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("last_polled_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("last_ok_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("last_error", sa.Text, nullable=True),
+        sa.Column("missing", sa.JSON, nullable=False, server_default="[]"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("snmp_devices")