fix(alerts): add AGENT_LIFECYCLE to condition_kind_enum + readable error toasts

Backend
 - V18 migration adds AGENT_LIFECYCLE to condition_kind_enum. Java
   ConditionKind enum shipped with this value but no Postgres migration
   extended the type, so any AGENT_LIFECYCLE rule insert failed with
   "invalid input value for enum condition_kind_enum".
 - ALTER TYPE ... ADD VALUE lives alone in its migration per Postgres
   constraint that the new value cannot be referenced in the same tx.
 - V18MigrationIT asserts the enum now contains all 7 kinds.

Frontend
 - Add describeApiError(e) helper to unwrap openapi-fetch error bodies
   (Spring error JSON) into readable strings. String(e) on a plain
   object rendered "[object Object]" in toasts — the actual failure
   reason was hidden from the user.
 - Replace String(e) in all 13 toast descriptions across the alerting
   and outbound-connection mutation paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/Alerts/SilencesPage.tsx

@@ -13,6 +13,7 @@ import {
   useDeleteSilence,
   type AlertSilenceResponse,
 } from '../../api/queries/alertSilences';
+import { describeApiError } from '../../api/errors';
 import sectionStyles from '../../styles/section-card.module.css';
 import tableStyles from '../../styles/table-section.module.css';
 import css from './alerts-page.module.css';
@@ -63,7 +64,7 @@ export default function SilencesPage() {
       setHours(1);
       toast({ title: 'Silence created', variant: 'success' });
     } catch (e) {
-      toast({ title: 'Create failed', description: String(e), variant: 'error' });
+      toast({ title: 'Create failed', description: describeApiError(e), variant: 'error' });
     }
   };
 
@@ -73,7 +74,7 @@ export default function SilencesPage() {
       await remove.mutateAsync(pendingEnd.id!);
       toast({ title: 'Silence removed', variant: 'success' });
     } catch (e) {
-      toast({ title: 'Remove failed', description: String(e), variant: 'error' });
+      toast({ title: 'Remove failed', description: describeApiError(e), variant: 'error' });
     } finally {
       setPendingEnd(null);
     }