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>

cameleer-server-app/src/test/java/com/cameleer/server/app/alerting/storage/V18MigrationIT.java

@@ -0,0 +1,20 @@
+package com.cameleer.server.app.alerting.storage;
+
+import com.cameleer.server.app.AbstractPostgresIT;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class V18MigrationIT extends AbstractPostgresIT {
+
+    @Test
+    void condition_kind_enum_includes_agent_lifecycle() {
+        var values = jdbcTemplate.queryForList("""
+            SELECT unnest(enum_range(NULL::condition_kind_enum))::text AS v
+            """, String.class);
+        assertThat(values).contains("AGENT_LIFECYCLE");
+        assertThat(values).containsExactlyInAnyOrder(
+            "ROUTE_METRIC", "EXCHANGE_MATCH", "AGENT_STATE", "AGENT_LIFECYCLE",
+            "DEPLOYMENT_STATE", "LOG_PATTERN", "JVM_METRIC");
+    }
+}