feat: real plugin system with loadable instances + OpenBao secrets (v1.17.0)

Generalize the half-built discovery plugins into a real plugin system: plugin
TYPES (the plugins/<category>/<type>.js modules with manifests) and loadable,
configurable, multi-copy plugin INSTANCES (PluginInstance ORM model) managed
from a dedicated /plugins page and /api/plugins API, with per-instance secrets
in OpenBao at secret/plugins/<id>/conf.

- plugin_registry.js: getTypes/getModule/splitConfig/mask + required-field helpers
- PluginInstance model (Sequelize): id/pluginType/category/name/slug(unique)/
  enabled/cron/config(json, non-secret)/lastRun*; registered in models/index.js
- plugin_secrets.js: read/write/remove/mergeForRun over @simpleworkjs/bao-conf
- scheduler.js: schedules from the DB registry; per-instance stable BullMQ
  JobScheduler ids (plugin:<id>) for load/unload; legacy migration from
  conf.discovery.plugins on first boot (idempotent, empty-table-guarded)
- api_plugins.js (replaces routes/plugins.js): types/list/get/create/update/
  secrets/test/load/unload/run/delete/runs; admin-gated; secrets always masked
- /plugins page (plugins.ejs) + nav; Agents & Scheduler tab removed from
  /directory; /docs/agents aliased to /docs/plugins
- proxmox/unifi/nmap gained manifests (configSchema/validate/run alias)
- tests/plugins.test.js: registry unit + plugin_secrets (mocked bao-conf) +
  PluginInstance model round-trip/unique-slug
- docs (plugins.md, vault.md, _config.yml, API.md) + 1.16.1 -> 1.17.0

Requires theta-suite >= v1.30.1 for the sso-broker secret/plugins/* grant;
fails-soft with a clear error if absent.

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

nodejs/routes/docs.js

@@ -34,7 +34,8 @@ const DOCS = {
 	'oauth-apps':    {title: 'Connecting Apps (SSO)',   file: path.join(__dirname, '../../docs/concepts-oauth-apps.md')},
 	'api-tokens':    {title: 'API Tokens',              file: path.join(__dirname, '../../docs/concepts-api-tokens.md')},
 	directory:       {title: 'Directory & Inventory',   file: path.join(__dirname, '../../docs/directory.md')},
-	agents:          {title: 'Agents & Scheduler',      file: path.join(__dirname, '../../docs/agents.md')},
+	agents:          {title: 'Plugins',                file: path.join(__dirname, '../../docs/plugins.md')},
+	plugins:         {title: 'Plugins',                file: path.join(__dirname, '../../docs/plugins.md')},
 	vault:           {title: 'Vault Secrets',           file: path.join(__dirname, '../../docs/vault.md')},
 
 	overview:        {title: 'Overview',        file: path.join(__dirname, '../../README.md')},