YT Music-koppling i settings: Googles device-flöde inbyggt

- POST /api/ytmusic/start + /poll: kör device-flödet mot Google och
  skriver ytmusicapi-kompatibel oauth.json till Youtubarrs datamapp
  (mounten byts till rw i stacken)
- Settings-sektion: Client ID/secret, 'Spara & koppla' visar kod + länk
  och pollar tills godkänt; 'Testa kopplingen' verifierar med en
  token-förnyelse mot Google
- 4 nya tester, totalt 21

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01824ZrvG2mDYYrmwqypLNup

backend/src/app.js

@@ -11,6 +11,7 @@ import { mbClient } from './musicbrainz.js'
 import { mapLidarrSearch } from './mapping.js'
 import { settingsStore } from './settings.js'
 import { makeTesters } from './testers.js'
+import { ytmusicFlow } from './ytmusic.js'
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url))
 
@@ -28,6 +29,7 @@ export function buildApp(config, deps = {}) {
   const mb = deps.musicbrainz ?? mbClient()
   const store = deps.store ?? settingsStore(config.dataDir ?? '/data')
   const testers = deps.testers ?? makeTesters({ config, store })
+  const ytmusic = deps.ytmusic ?? ytmusicFlow({ config, store })
   const app = Fastify({ logger: true, trustProxy: true })
 
   app.register(fastifyCookie)
@@ -157,6 +159,11 @@ export function buildApp(config, deps = {}) {
         userId: s.jellyfin?.userId ?? '',
         userName: s.jellyfin?.userName ?? '',
       },
+      ytmusic: {
+        clientIdSet: Boolean(s.ytmusic?.clientId),
+        clientSecretSet: Boolean(s.ytmusic?.clientSecret),
+        connected: existsSync(ytmusic.oauthPath),
+      },
     }
   })
 
@@ -171,10 +178,36 @@ export function buildApp(config, deps = {}) {
         userId: body.jellyfin?.userId,
         userName: body.jellyfin?.userName,
       },
+      ytmusic: {
+        clientId: body.ytmusic?.clientId,
+        clientSecret: body.ytmusic?.clientSecret,
+      },
     })
     return { ok: true }
   })
 
+  app.post('/api/ytmusic/start', async (req, reply) => {
+    try {
+      const out = await ytmusic.start()
+      if (out.error) return reply.code(400).send(out)
+      return out
+    } catch (err) {
+      req.log.error({ err }, 'device-flödet kunde inte startas')
+      return reply.code(502).send({ error: 'kunde inte nå Google' })
+    }
+  })
+
+  app.post('/api/ytmusic/poll', async (req, reply) => {
+    const deviceCode = req.body?.deviceCode
+    if (!deviceCode) return reply.code(400).send({ error: 'deviceCode saknas' })
+    try {
+      return await ytmusic.poll(deviceCode)
+    } catch (err) {
+      req.log.error({ err }, 'device-poll misslyckades')
+      return reply.code(502).send({ error: 'kunde inte nå Google' })
+    }
+  })
+
   app.post('/api/settings/test', async (req, reply) => {
     const service = req.body?.service
     if (!testers[service]) return reply.code(400).send({ error: 'okänd tjänst' })