feat(alerts): renames carry their alerts — monitor and host renames move rules, windows and history (#5065)

#5065 "Renames carry their alerts". Alert rules, maintenance-window scopes
and metric history key a thing by (source, name), so a rename used to leave
the rules watching a name nothing reported under, and split the history.

- core/renames.carry_rename moves AlertRule.resource_name,
  MaintenanceWindow.scope_resource, plugin_metrics and plugin_metrics_hourly
  to the new name in the caller's transaction. A host rename also carries
  its parts: host_agent "<host>:…" and docker "<host>/…" (HOST_SOURCES).
  Matching is exact (a prefix compare, not LIKE or a collation range).
  History left under the new name by something deleted is dropped first
  rather than merged in.
- Web monitor edit, MCP update_monitor and web host edit call it; MCP
  answers with `carried` (it used to return a warning naming stranded rules).
- A name another monitor of the same type, or another host, already has is
  refused on create and rename: two would share one history.
- The rename is audited (monitor.updated / host.renamed) with what moved.

Tests: an integration test renames a firing monitor through the web route
and finds its rule still firing and cleared by a sample on the new name,
its window and raw+hourly history moved and an orphan's history dropped; a
host rename moves agent and container parts but not a longer-named host's
or another source's; a guard that every host_agent resource is "<host>" or
"<host>:…".

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

steward/hosts/service.py

@@ -10,6 +10,7 @@ from datetime import datetime, timedelta, timezone
 from sqlalchemy import and_, case, func, select
 
 from steward.core.status import _heartbeat_title, _last_n_results, _uptime_by_monitor
+from steward.models.hosts import Host
 from steward.models.monitors import Monitor, MonitorResult
 
 
@@ -18,6 +19,21 @@ from steward.models.monitors import Monitor, MonitorResult
 AGENT_GLANCE_METRICS = ("cpu_pct", "mem_used_pct", "disk_used_pct", "disk_used_pct_worst")
 
 
+async def check_name_free(db, name: str, exclude_id: str | None = None) -> None:
+    """ValueError when another host already has this name, or it is blank.
+
+    Agent metrics, container metrics and the alert rules on them go by the
+    host's name, so two hosts sharing one would share one history.
+    """
+    if not name:
+        raise ValueError("a host needs a name.")
+    q = select(Host.id).where(Host.name == name)
+    if exclude_id:
+        q = q.where(Host.id != exclude_id)
+    if (await db.execute(q.limit(1))).first():
+        raise ValueError(f"another host is already named {name!r}; pick a different name.")
+
+
 def agent_stale_after_seconds(config) -> int:
     """The host_agent plugin's freshness window from app config (default 180s)."""
     return int(((config.get("PLUGINS") or {}).get("host_agent") or {})