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/main/resources/db/migration/V18__condition_kind_add_agent_lifecycle.sql

@@ -0,0 +1,10 @@
+-- V18 — Add AGENT_LIFECYCLE to condition_kind_enum.
+--
+-- The Java ConditionKind enum shipped AGENT_LIFECYCLE with the alerting
+-- branch, but no migration ever extended the Postgres type. Inserting a
+-- rule with conditionKind=AGENT_LIFECYCLE failed with
+--   ERROR: invalid input value for enum condition_kind_enum: "AGENT_LIFECYCLE"
+-- ALTER TYPE ... ADD VALUE must live alone in its migration — Postgres won't
+-- allow the new value to be referenced in the same transaction that adds it.
+
+ALTER TYPE condition_kind_enum ADD VALUE IF NOT EXISTS 'AGENT_LIFECYCLE' AFTER 'AGENT_STATE';