Persist Plutonium keys by server ID and add key management

src/lib/server/db/schema.sql

@@ -238,6 +238,13 @@ CREATE TABLE IF NOT EXISTS deployment_scripts (
 );
 CREATE INDEX IF NOT EXISTS idx_deployment_scripts_dep ON deployment_scripts(deployment_id);
 
+-- Deliberately independent of servers: credentials survive server destruction.
+CREATE TABLE IF NOT EXISTS server_keys (
+    server_id TEXT PRIMARY KEY,
+    pluto_key TEXT NOT NULL,
+    updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
 CREATE TABLE IF NOT EXISTS settings (
     key   TEXT PRIMARY KEY,
     value TEXT NOT NULL