Add Alerts webapp UI (ticket 020)

New Alerts panel: lists rules with condition summaries and an enabled
toggle, and a condition-type-specific editor (Immediate/Threshold/NewValue/
Ratio) with client-side validation before save, reusing FilterBar's query
input styling for query-based conditions. Adds a test-notification endpoint
(POST /api/settings/alerts/test) so an ntfy topic/server can be verified
without saving a rule first, plus a docs section and help icons matching
ticket 021's pattern.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Docs.tsx

@@ -233,6 +233,51 @@ const SECTIONS: DocSection[] = [
       </div>
     ),
   },
+  {
+    id: 'alerts',
+    title: 'Alerts',
+    body: (
+      <div className="flex flex-col gap-2">
+        <p>
+          An alert rule fires a notification when its condition matches a newly-ingested log line — evaluated per
+          line as it arrives, not backfilled against history. There are four condition types:
+        </p>
+        <ul className="list-disc pl-5">
+          <li>
+            <span className="font-semibold text-text">Immediate</span> — fires once per line matching a query.
+          </li>
+          <li>
+            <span className="font-semibold text-text">Threshold</span> — fires when a query's match count within a
+            rolling window crosses a numeric threshold.
+          </li>
+          <li>
+            <span className="font-semibold text-text">NewValue</span> — fires the first time a field regex captures a
+            value not seen before by this rule (seen values live only in memory, so a restart forgets them).
+          </li>
+          <li>
+            <span className="font-semibold text-text">Ratio</span> — fires when the ratio of two queries' match
+            counts within a rolling window crosses a numeric ratio.
+          </li>
+        </ul>
+        <p>
+          Query-based conditions (Immediate/Threshold/Ratio) reuse the filter bar's query syntax, but only the subset
+          that evaluates against a single line's own fields — <Code>regex(...)</Code>, <Code>sequence</Code>,{' '}
+          <Code>timestamp</Code>, and <Code>and</Code>/<Code>or</Code>. A <Code>metric_name[param]</Code> comparison
+          isn't supported in an alert query.
+        </p>
+        <p>
+          A window accepts a plain number of milliseconds or a humantime-style duration like <Code>30s</Code>,{' '}
+          <Code>5m</Code>, <Code>1h</Code>.
+        </p>
+        <p>
+          Every rule delivers to an <em>ntfy</em> sink: a topic (required) and an optional server override (defaults
+          to <Code>https://ntfy.sh</Code>). "Send test notification" delivers to whatever topic/server is currently
+          entered in the form, without needing to save the rule first — handy for verifying the sink before
+          committing to it.
+        </p>
+      </div>
+    ),
+  },
   {
     id: 'settings',
     title: 'Settings',