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/plugin.yaml

@@ -1,7 +1,7 @@
 # plugins/snmp/plugin.yaml
 name: snmp
-version: "1.0.0"
-description: "SNMP polling for network devices — switches, routers, printers, UPSes, etc."
+version: "2.0.0"
+description: "SNMP polling for network devices — switches, routers, printers, UPSes, servers."
 author: "Steward"
 license: "MIT"
 min_app_version: "0.1.0"
@@ -12,48 +12,6 @@ tags:
   - network
   - monitoring
 
+# Devices are added, tested and edited on the SNMP page (/plugins/snmp/).
 config:
   poll_interval_seconds: 60
-  # Each device is polled independently. OIDs must resolve to numeric values.
-  #
-  # `host` is the SNMP poll target (IP/hostname we query). A device also shows
-  # up on a Steward Host's detail page when it maps to that host. By default
-  # that mapping is implicit: `host` is matched against the Host's address or
-  # name. To bind explicitly instead — e.g. when you poll by IP but the Host is
-  # recorded by DNS name, or for an SNMP-only switch/PDU/UPS — add ONE of:
-  #   host_id:      "<steward-host-uuid>"   # exact Host.id match
-  #   steward_host: "switch01"              # Host name OR address (case-insensitive)
-  # An explicit binding is exclusive: the device then maps ONLY to that host,
-  # never implicitly to another by a coincidental address string.
-  devices:
-    - name: "core-switch"
-      host: "192.168.1.1"
-      port: 161
-      community: "public"
-      version: "2c"    # "1", "2c", or "3"
-      oids:
-        - oid: "1.3.6.1.2.1.1.3.0"
-          label: "uptime_centisecs"
-        - oid: "1.3.6.1.2.1.2.2.1.10.1"
-          label: "if1_in_octets"
-        - oid: "1.3.6.1.2.1.2.2.1.16.1"
-          label: "if1_out_octets"
-    # Example: network UPS with SNMP management card (RFC 1628 UPS-MIB).
-    # upsBatteryStatus values: 1=unknown, 2=batteryNormal, 3=batteryLow, 4=batteryDepleted.
-    # Uncomment and point at your UPS to enable.
-    # - name: "ups"
-    #   host: "192.168.1.10"
-    #   port: 161
-    #   community: "public"
-    #   version: "2c"
-    #   oids:
-    #     - oid: "1.3.6.1.2.1.33.1.2.1.0"
-    #       label: "battery_status"
-    #     - oid: "1.3.6.1.2.1.33.1.2.2.0"
-    #       label: "seconds_on_battery"
-    #     - oid: "1.3.6.1.2.1.33.1.2.3.0"
-    #       label: "runtime_minutes_remaining"
-    #     - oid: "1.3.6.1.2.1.33.1.2.4.0"
-    #       label: "battery_charge_pct"
-    #     - oid: "1.3.6.1.2.1.33.1.3.3.1.3.1"
-    #       label: "input_voltage_dV"