Add canStream user permission to control streaming access

Adds a per-user "Can Stream" permission mirroring the existing "Can Download"
pattern. Server admins can now disable streaming for specific users, encouraging
local downloads instead. Addresses #2572.

Changes:
- User model: stream permission in mapping, defaults, and getter
- ApiKey model: stream permission in defaults
- Controller: 403 enforcement on playback session creation endpoints
- Frontend: permission toggle in admin UI, play button gated by canStream,
  download button shown when streaming disabled, message when neither allowed
- Tests: 11 Mocha tests (model + controller), 1 Cypress test (card UI)
- Localization: English strings for toggle label and fallback message

The getter uses !== false (rather than !!) so existing users without the
stream key in their permissions JSON default to allowed on upgrade.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/server/controllers/LibraryItemController.canStream.test.js

@@ -0,0 +1,101 @@
+const { expect } = require('chai')
+const sinon = require('sinon')
+
+const LibraryItemController = require('../../../server/controllers/LibraryItemController')
+const Logger = require('../../../server/Logger')
+
+describe('LibraryItemController - canStream enforcement', () => {
+  beforeEach(() => {
+    sinon.stub(Logger, 'warn')
+    sinon.stub(Logger, 'error')
+  })
+
+  afterEach(() => {
+    sinon.restore()
+  })
+
+  describe('startPlaybackSession', () => {
+    it('should return 403 when user cannot stream', () => {
+      const req = {
+        user: { canStream: false, username: 'testuser' },
+        libraryItem: { hasAudioTracks: true, id: 'li_test' }
+      }
+      const res = { sendStatus: sinon.spy() }
+
+      LibraryItemController.startPlaybackSession.call({}, req, res)
+
+      expect(res.sendStatus.calledWith(403)).to.be.true
+      expect(Logger.warn.calledOnce).to.be.true
+      expect(Logger.warn.firstCall.args[0]).to.include('testuser')
+    })
+
+    it('should not block when user can stream', () => {
+      const startSessionRequest = sinon.spy()
+      const req = {
+        user: { canStream: true },
+        libraryItem: { hasAudioTracks: true, id: 'li_test' }
+      }
+      const res = { sendStatus: sinon.spy() }
+
+      LibraryItemController.startPlaybackSession.call(
+        { playbackSessionManager: { startSessionRequest } },
+        req,
+        res
+      )
+
+      expect(res.sendStatus.called).to.be.false
+      expect(startSessionRequest.calledOnce).to.be.true
+    })
+
+    it('should return 404 when item has no audio tracks', () => {
+      const req = {
+        user: { canStream: true },
+        libraryItem: { hasAudioTracks: false, id: 'li_test' }
+      }
+      const res = { sendStatus: sinon.spy() }
+
+      LibraryItemController.startPlaybackSession.call({}, req, res)
+
+      expect(res.sendStatus.calledWith(404)).to.be.true
+    })
+  })
+
+  describe('startEpisodePlaybackSession', () => {
+    it('should return 403 when user cannot stream', () => {
+      const req = {
+        user: { canStream: false, username: 'testuser' },
+        libraryItem: { isPodcast: true, id: 'li_test' },
+        params: { episodeId: 'ep_1' }
+      }
+      const res = { sendStatus: sinon.spy() }
+
+      LibraryItemController.startEpisodePlaybackSession.call({}, req, res)
+
+      expect(res.sendStatus.calledWith(403)).to.be.true
+      expect(Logger.warn.calledOnce).to.be.true
+    })
+
+    it('should not block when user can stream', () => {
+      const startSessionRequest = sinon.spy()
+      const req = {
+        user: { canStream: true },
+        libraryItem: {
+          isPodcast: true,
+          id: 'li_test',
+          media: { podcastEpisodes: [{ id: 'ep_1' }] }
+        },
+        params: { episodeId: 'ep_1' }
+      }
+      const res = { sendStatus: sinon.spy() }
+
+      LibraryItemController.startEpisodePlaybackSession.call(
+        { playbackSessionManager: { startSessionRequest } },
+        req,
+        res
+      )
+
+      expect(res.sendStatus.called).to.be.false
+      expect(startSessionRequest.calledOnce).to.be.true
+    })
+  })
+})