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/routes/api/settings/+server.ts

@@ -0,0 +1,12 @@
+import { json } from '@sveltejs/kit';
+import { validateFleetSettings } from '$lib/fleet-settings.js';
+import { fleetSettings, saveFleetSettings } from '$lib/server/settings.js';
+
+export function GET() { return json(fleetSettings()); }
+
+export async function PUT({ request }: { request: Request }) {
+    let settings;
+    try { settings = validateFleetSettings(await request.json()); }
+    catch (error) { return json({ error: (error as Error).message }, { status: 400 }); }
+    return json(saveFleetSettings(settings));
+}