chore(psr4): phase 3 — namespace Domain layer

Namespace all of Domain into XcVm\Domain\<Subdir> (33 classes across Bouquet,
Device, Epg, Line, Security, Server, Stream, User, Vod).

- Add namespace to every Domain class; add use to referrers across src/ and
  tests/; convert the leading-backslash refs qualified in earlier Core commits
  (\UserRepository, \ServerRepository, \BouquetService, \CategoryService,
  \ConnectionTracker, \BlocklistService, ...) to their FQCNs.
- Qualify still-global / built-in deps inside Domain with leading backslash
  (\RedisManager, \TMDB, \WatchService, \FFprobeRunner, \ProcessChecker,
  \Exception, \DateTime, \PDO, ...) — Infrastructure/Streaming/module classes
  migrate later.
- BruteforceGuard: string guards class_exists('ServerRepository'/'BlocklistService')
  → ::class FQCN; drop the now-always-true method_exists check.
- phpstan-baseline.neon regenerated (292→291).

Verified: php -l clean; PHPStan no errors; PHPUnit 295/295; leading-backslash
re-sweep across Domain classes is clean.

tests/Unit/StreamConfigRepositoryTest.php

@@ -1,5 +1,6 @@
 <?php
 
+use XcVm\Domain\Stream\StreamConfigRepository;
 use PHPUnit\Framework\TestCase;
 
 /**