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/RuleEditor/TriggerStep.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { Button, FormField, Input, useToast } from '@cameleer/design-system';
 import { useTestEvaluate } from '../../../api/queries/alertRules';
+import { describeApiError } from '../../../api/errors';
 import type { FormState } from './form-state';
 
 export function TriggerStep({
@@ -25,7 +26,7 @@ export function TriggerStep({
       const result = await testEvaluate.mutateAsync({ id: ruleId, req: {} });
       setLastResult(JSON.stringify(result, null, 2));
     } catch (e) {
-      toast({ title: 'Test-evaluate failed', description: String(e), variant: 'error' });
+      toast({ title: 'Test-evaluate failed', description: describeApiError(e), variant: 'error' });
     }
   };