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

steward/core/widgets.py

@@ -12,15 +12,50 @@
 from __future__ import annotations
 
 WIDGET_REGISTRY: dict[str, dict] = {
-    "ping": {
-        "key": "ping",
-        "label": "Ping",
-        "description": "Live ping status and latency history for all monitored hosts",
-        "hx_url": "/ping/rows",
-        "detail_url": "/ping/",
+    "monitors": {
+        "key": "monitors",
+        "label": "Monitors",
+        "description": "Uptime checks of every type — ping, DNS, and HTTP — with up/down status and latency",
+        "hx_url": "/monitors/widget",
+        "detail_url": "/monitors/",
         "plugin": None,
         "poll": True,
-        "params": [],
+        "params": [
+            {
+                "key": "type_filter",
+                "label": "Type",
+                "type": "select",
+                "default": "all",
+                "options": [
+                    {"value": "all",  "label": "All types"},
+                    {"value": "icmp", "label": "Ping (ICMP)"},
+                    {"value": "tcp",  "label": "Ping (TCP)"},
+                    {"value": "dns",  "label": "DNS"},
+                    {"value": "http", "label": "HTTP"},
+                ],
+            },
+            {
+                "key": "show_down_only",
+                "label": "Display filter",
+                "type": "select",
+                "default": "no",
+                "options": [
+                    {"value": "no",  "label": "All monitors"},
+                    {"value": "yes", "label": "Failing only"},
+                ],
+            },
+            {
+                "key": "limit",
+                "label": "Max shown",
+                "type": "select",
+                "default": 10,
+                "options": [
+                    {"value": 5,  "label": "5 monitors"},
+                    {"value": 10, "label": "10 monitors"},
+                    {"value": 20, "label": "20 monitors"},
+                ],
+            },
+        ],
     },
     "status_overview": {
         "key": "status_overview",
@@ -52,16 +87,6 @@ WIDGET_REGISTRY: dict[str, dict] = {
         "poll": False,
         "params": [],
     },
-    "dns": {
-        "key": "dns",
-        "label": "DNS",
-        "description": "DNS resolution status for all monitored hosts",
-        "hx_url": "/dns/rows",
-        "detail_url": "/dns/",
-        "plugin": None,
-        "poll": True,
-        "params": [],
-    },
     "traefik_routers": {
         "key": "traefik_routers",
         "label": "Traefik — Routers",
@@ -203,38 +228,6 @@ WIDGET_REGISTRY: dict[str, dict] = {
             },
         ],
     },
-    "http_monitors": {
-        "key": "http_monitors",
-        "label": "HTTP Monitors",
-        "description": "Synthetic endpoint checks — up/down status and response times",
-        "hx_url": "/plugins/http/widget",
-        "detail_url": "/plugins/http/",
-        "plugin": "http",
-        "poll": True,
-        "params": [
-            {
-                "key": "show_down_only",
-                "label": "Display filter",
-                "type": "select",
-                "default": "no",
-                "options": [
-                    {"value": "no",  "label": "All monitors"},
-                    {"value": "yes", "label": "Failing only"},
-                ],
-            },
-            {
-                "key": "limit",
-                "label": "Max shown",
-                "type": "select",
-                "default": 10,
-                "options": [
-                    {"value": 5,  "label": "5 monitors"},
-                    {"value": 10, "label": "10 monitors"},
-                    {"value": 20, "label": "20 monitors"},
-                ],
-            },
-        ],
-    },
     "docker_resources": {
         "key": "docker_resources",
         "label": "Docker — Resources",
@@ -312,7 +305,7 @@ WIDGET_REGISTRY: dict[str, dict] = {
 # Group each widget for the picker (Core monitors / Monitoring capabilities /
 # Integrations), mirroring the Settings → Plugins taxonomy. Capability plugins
 # are host/monitoring facets; traefik/unifi are discrete vendor integrations.
-_CORE_WIDGETS = {"ping", "dns", "status_overview", "uptime_summary", "hosts_overview"}
+_CORE_WIDGETS = {"monitors", "status_overview", "uptime_summary", "hosts_overview"}
 _INTEGRATION_PLUGINS = {"traefik", "unifi"}
 GROUP_LABELS = {
     "core": "Core monitors",