feat(alerts): a sentence per alert, a link to what it watches, a count in the sidebar (#5124)

The Alerts page's active list now says each alert in one line around the
resource it watches, e.g. "cpu_pct on web-1 is 94% (> 90%), firing for 12m",
with web-1 linking to its host, monitor or container page (or the plugin's
page for Traefik, UniFi and SNMP). Values take their unit from the metric
name through formatting.metric_value. A silent series reads "No cpu_pct from
web-1 for 20m"; an up/down metric says the word.

Actions follow the routes' roles: acknowledge, edit, toggle and New rule for
operators and admins, Delete for admins only, nothing for viewers.

The sidebar shows how many alerts need attention beside Alerts, from an
async context processor that skips htmx fragments, logged-out pages and the
database-down page, and renders without the count if the query fails.

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

steward/static/steward.css

@@ -86,6 +86,12 @@ code { font-family: var(--fs-font-mono); font-size: 0.875em; background: var(--b
   color: var(--gold); border-left-color: var(--gold);
   background: color-mix(in srgb, var(--gold) 8%, transparent);
 }
+/* How many alerts need attention, beside the Alerts link. */
+.nav-count {
+  margin-left: auto; padding: 0 0.45em; border-radius: var(--fs-radius-sm);
+  background: var(--red-dim); color: var(--red); font-size: 0.75rem; font-weight: 600;
+  font-variant-numeric: tabular-nums;
+}
 .side-user { border-top: 1px solid var(--border); padding: 0.7rem 1.25rem; }
 .side-user form { margin: 0; }
 .side-user button {