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/__init__.py

@@ -11,6 +11,7 @@ _app: "Quart | None" = None
 def setup(app: "Quart") -> None:
     global _app
     _app = app
+    from .models import SnmpDevice  # noqa: F401 — registers the table
 
 
 def get_scheduled_tasks() -> list:
@@ -32,3 +33,9 @@ def get_alert_metrics() -> dict[str, list[str]]:
     """SNMP metric names are the OID labels each device is configured with, so
     the list is empty here and the rule form offers the labels recorded so far."""
     return {"snmp": []}
+
+
+def get_audit_entities() -> dict:
+    """Audit rows about a device name it and link to its page."""
+    from .models import SnmpDevice
+    return {"snmp_device": (SnmpDevice, "/plugins/snmp/devices/{}")}