feat(db): cluster_nodes table for multi-server registry

services/mam-api/src/db/migrations/004-cluster-nodes.sql

@@ -0,0 +1,15 @@
+CREATE TABLE IF NOT EXISTS cluster_nodes (
+  id            UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+  hostname      TEXT         NOT NULL,
+  ip_address    TEXT,
+  role          TEXT         NOT NULL DEFAULT 'worker',
+  version       TEXT,
+  api_url       TEXT,
+  cpu_usage     NUMERIC(5,2),
+  mem_used_mb   INTEGER,
+  mem_total_mb  INTEGER,
+  last_seen     TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  registered_at TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  metadata      JSONB,
+  CONSTRAINT cluster_nodes_hostname_uq UNIQUE (hostname)
+);