feat(monitors): unify ping/dns/http into one Monitor entity + custom targets

Collapse the three former check types into a single core `Monitor` entity
with one management surface (/monitors), one result table (monitor_results),
and a single scheduled task. Every type can now watch a free-standing custom
destination (optional host_id) — not just a registered Host.

- models: Monitor + MonitorResult replace PingResult/DnsResult; Host loses its
  ping/dns facet columns (now Monitor rows linked by host_id).
- checks: monitors/{ping,dns,http}.py pure probes + runner.run_monitor
  dispatcher; one monitor_check scheduler with a per-monitor due-filter.
- status: single monitor_status_source replaces the three sources.
- UI: /monitors blueprint (type-aware add/edit/list/widget); host hub shows a
  host's linked monitors + "add monitor for this host"; nav + widget registry
  + alert metric catalog rewired. http plugin folded into core and removed.
- migration 0022 merges the http branch, data-migrates host facets +
  http_monitors + all three result histories, drops the old tables/columns.

Resolves the per-host ping/dns auto-attach issue (#275): monitors are now
explicit, never auto-added to every host.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/index.yaml

@@ -13,22 +13,6 @@ updated: "2026-03-22"
 
 plugins:
 
-  - name: http
-    version: "1.0.0"
-    description: "Synthetic HTTP endpoint monitoring — status code, response time, content match, TLS expiry"
-    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/http"
-    download_url: "https://git.fabledsword.com/bvandeusen/Steward-plugins/releases/download/http-v1.0.0/http.zip"
-    checksum_sha256: ""
-    tags:
-      - monitoring
-      - http
-      - synthetic
-      - uptime
-
   - name: docker
     version: "1.0.0"
     description: "Docker container status, resource usage, and restart tracking via Docker socket"