refactor(plugins): no plugin catalog; plugins ship in the image or are mounted (#5374)

First-party plugins have shipped inside the image since a7a281c, and the
catalog's only repository (Steward-plugins) answers 404, so Steward polled a
dead URL every minute for nothing. The operator chose to remove the catalog
entirely (#5214 "Review: readable and fast on the live instance").

Removed:
- core/plugin_index.py and the plugin_catalog_refresh startup task;
- the plugins.index_url default, and migration 0035 deletes the stored
  plugins.index_url / plugins.repositories rows;
- the Settings -> Plugins repository editor, catalog, refresh, install and
  reload routes and their templates;
- the download/checksum/extract install path in plugin_manager, and the
  PLUGIN_INSTALL_DIR config it wrote to;
- plugins/index.yaml, docs/plugins/index.yaml.example and the guide's
  "Publishing to the Catalog" section (now "Shipping a Plugin").

Kept: enabling and configuring plugins, hot-reload on enable, restart, orphan
cleanup, and the external root (/data/plugins) for operator-mounted plugins.
tests/test_no_plugin_catalog.py guards the routes, default and task.

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

plugins/index.yaml

@@ -1,74 +0,0 @@
-# steward-plugins / index.yaml
-#
-# Plugin catalog for Steward.
-# Fetched by the app at: Settings → Plugins → Plugin Catalog
-#
-# After updating an entry, commit and push — changes are live within 5 minutes
-# (the app caches this file in-process for 300 seconds).
-#
-# See docs/plugins/index.yaml.example in the main app repo for the full schema.
-
-version: 1
-updated: "2026-03-22"
-
-plugins:
-
-  - name: docker
-    version: "2.0.0"
-    description: "Per-host Docker container status + resource usage, collected by the Steward host agent"
-    author: "Steward"
-    license: "MIT"
-    min_app_version: "0.1.0"
-    repository_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins"
-    homepage: "https://git.fabledsword.com/bvandeusen/Steward-plugins/src/branch/main/docker"
-    download_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins/releases/download/docker-v2.0.0/docker.zip"
-    checksum_sha256: ""
-    tags:
-      - containers
-      - docker
-      - infrastructure
-
-  - name: traefik
-    version: "1.0.0"
-    description: "Traefik reverse proxy metrics and access log integration"
-    author: "Steward"
-    license: "MIT"
-    min_app_version: "0.1.0"
-    repository_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins"
-    homepage: "https://git.fabledsword.com/bvandeusen/Steward-plugins/src/branch/main/traefik"
-    download_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins/releases/download/traefik-v1.0.0/traefik.zip"
-    checksum_sha256: ""
-    tags:
-      - proxy
-      - metrics
-      - access-log
-
-  - name: unifi
-    version: "1.0.0"
-    description: "UniFi Network controller integration — WAN health, devices, clients, DPI"
-    author: "Steward"
-    license: "MIT"
-    min_app_version: "0.1.0"
-    repository_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins"
-    homepage: "https://git.fabledsword.com/bvandeusen/Steward-plugins/src/branch/main/unifi"
-    download_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins/releases/download/unifi-v1.0.0/unifi.zip"
-    checksum_sha256: ""
-    tags:
-      - network
-      - unifi
-      - ubiquiti
-
-  - name: ups
-    version: "1.0.0"
-    description: "UPS monitoring via NUT (Network UPS Tools) with Ansible shutdown automation"
-    author: "Steward"
-    license: "MIT"
-    min_app_version: "0.1.0"
-    repository_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins"
-    homepage: "https://git.fabledsword.com/bvandeusen/Steward-plugins/src/branch/main/ups"
-    download_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins/releases/download/ups-v1.0.0/ups.zip"
-    checksum_sha256: ""
-    tags:
-      - ups
-      - power
-      - nut