Move fleet retention and concurrency controls into Settings

Persist settings in SQLite, import legacy environment values once, and apply changes to cleanup and deployment queues without restarting.

Closes #51

src/lib/server/settings.ts

@@ -0,0 +1,14 @@
+import { validateFleetSettings, type FleetSettings } from '../fleet-settings.js';
+import { db } from './db/index.js';
+import { FLEET_SETTINGS_KEY } from './db/fleet-settings.js';
+
+export function fleetSettings(): FleetSettings {
+    const row = db().prepare('SELECT value FROM settings WHERE key = ?').get(FLEET_SETTINGS_KEY) as { value: string };
+    return JSON.parse(row.value);
+}
+
+export function saveFleetSettings(input: unknown): FleetSettings {
+    const settings = validateFleetSettings(input);
+    db().prepare('UPDATE settings SET value = ? WHERE key = ?').run(JSON.stringify(settings), FLEET_SETTINGS_KEY);
+    return settings;
+}