feat: persistent per-application config with GET/PUT endpoints

Add application_config table (V4 migration), repository, and REST
controller. GET /api/v1/config/{app} returns config, PUT saves and
pushes CONFIG_UPDATE to all LIVE agents via SSE. UI tracing toggle
now uses config API instead of direct SET_TRACED_PROCESSORS command.
Tracing store syncs with server config on load.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/resources/db/migration/V4__application_config.sql

@@ -0,0 +1,9 @@
+-- Per-application configuration for agent observability settings.
+-- Agents download this at startup and receive updates via SSE CONFIG_UPDATE.
+CREATE TABLE application_config (
+    application   TEXT PRIMARY KEY,
+    config_val    JSONB NOT NULL,
+    version       INTEGER NOT NULL DEFAULT 1,
+    updated_at    TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_by    TEXT
+);