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/api/errors.ts

@@ -0,0 +1,24 @@
+/**
+ * Turn whatever a fetch/mutation threw into a user-readable string.
+ *
+ * openapi-fetch rethrows the parsed response body as-is (a plain object like
+ * `{ error, message, path, status, timestamp }` for Spring errors), so
+ * `String(e)` renders "[object Object]" in a toast. This helper prefers an
+ * Error.message, falls back to a `message` field on plain objects, then
+ * Spring's `error` field, and finally JSON-stringifies the rest.
+ */
+export function describeApiError(e: unknown): string {
+  if (e instanceof Error) return e.message;
+  if (typeof e === 'string') return e;
+  if (typeof e === 'object' && e !== null) {
+    const obj = e as Record<string, unknown>;
+    if (typeof obj.message === 'string' && obj.message) return obj.message;
+    if (typeof obj.error === 'string' && obj.error) return obj.error;
+    try {
+      return JSON.stringify(e);
+    } catch {
+      return String(e);
+    }
+  }
+  return String(e);
+}