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>

docs/plugins/writing-a-plugin.md

@@ -323,6 +323,21 @@ Role hierarchy: `admin > operator > viewer`. Requiring `viewer` grants access to
 
 ---
 
+## Audit Log Entries
+
+A change made through a plugin's pages is audited with `steward.core.audit.log_audit`, naming the thing it changed by `entity_type` and `entity_id`. So the audit page shows that thing's current name, linked to its page, export `get_audit_entities()` from `__init__.py`:
+
+```python
+# plugins/myplugin/__init__.py
+def get_audit_entities() -> dict:
+    from .models import Target
+    return {"myplugin_target": (Target, "/plugins/myplugin/targets/{}")}
+```
+
+The model needs `id` and `name` columns. An entity deleted since keeps the name the entry recorded in its detail (`detail={"name": ...}`). Add the action's wording to `ACTIONS` in `steward/audit/service.py` so the row reads as a sentence.
+
+---
+
 ## Agent Tools (MCP)
 
 Steward serves an MCP endpoint at `/mcp` that AI clients reach with a personal API token created on the AI access page. A plugin adds tools to it by exporting an optional `register_mcp_tools(register)` from `__init__.py`: