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/Admin/OutboundConnectionEditor.tsx

@@ -16,6 +16,7 @@ import {
   type TrustMode,
 } from '../../api/queries/admin/outboundConnections';
 import { useEnvironments } from '../../api/queries/admin/environments';
+import { describeApiError } from '../../api/errors';
 import sectionStyles from '../../styles/section-card.module.css';
 
 // ── Form state ──────────────────────────────────────────────────────────
@@ -145,12 +146,12 @@ export default function OutboundConnectionEditor() {
           toast({ title: 'Created', description: form.name, variant: 'success' });
           navigate('/admin/outbound-connections');
         },
-        onError: (e) => toast({ title: 'Create failed', description: String(e), variant: 'error' }),
+        onError: (e) => toast({ title: 'Create failed', description: describeApiError(e), variant: 'error' }),
       });
     } else {
       updateMut.mutate(payload, {
         onSuccess: () => toast({ title: 'Updated', description: form.name, variant: 'success' }),
-        onError: (e) => toast({ title: 'Update failed', description: String(e), variant: 'error' }),
+        onError: (e) => toast({ title: 'Update failed', description: describeApiError(e), variant: 'error' }),
       });
     }
   };