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>

server/models/ApiKey.js

@@ -6,6 +6,7 @@ const Logger = require('../Logger')
 /**
  * @typedef {Object} ApiKeyPermissions
  * @property {boolean} download
+ * @property {boolean} stream
  * @property {boolean} update
  * @property {boolean} delete
  * @property {boolean} upload
@@ -84,6 +85,7 @@ class ApiKey extends Model {
   static getDefaultPermissions() {
     return {
       download: true,
+      stream: true,
       update: true,
       delete: true,
       upload: true,