feat: QR code phone pairing for Decky plugin + MangoHud guide revision + SteamOS version fix

- Add plugin_pairings table + migration (0028) for short-lived pairing sessions
- Add pairing API: POST /api/plugin/pair/initiate, GET /status/:token, POST /confirm
- Add /pair web page: user scans QR on phone, confirms, API key auto-created
- Plugin: 'Pair with Phone' button renders QR code (qrcode.react), polls for status, saves API key
- Plugin: new initiate_pair + check_pair_status Python RPCs
- Revise MangoHud Setup Guide into clean numbered steps
- Fix SteamOS version detection: read VERSION_ID + BUILD_ID (was just 'SteamOS')

apps/web/lib/api/app.ts

@@ -52,6 +52,7 @@ import { eq } from "drizzle-orm"
 import { mobileRoutes } from "@/lib/api/mobile"
 import { gamesLookupRoutes } from "@/lib/api/games-lookup"
 import { performanceImportRoutes } from "@/lib/api/performance-import"
+import { pluginPairingRoutes } from "@/lib/api/plugin-pairing"
 
 const betterAuth = new Elysia({ name: "better-auth" })
   .mount(auth.handler)
@@ -245,6 +246,7 @@ export const app = new Elysia({ prefix: "/api" })
       .use(screenshotRoutes)
       .use(gamesLookupRoutes)
       .use(mobileRoutes)
+      .use(pluginPairingRoutes)
   )
   // ── Write routes ───────────────────────────────────────────
   .group("", (app) =>