fix(phpstan): fix real bugs (return contracts, arg/byref, stub types) 599→585

- constants stub: use mt_rand()-based exprs so PHPStan infers GENERAL types,
  not literal 0/'' — fixes false division-by-zero (PACKET_SIZE) and
  foreach-over-false (str_split with len 0). Load stub via bootstrapFiles so
  result-cache invalidates on change.
- return contracts: explicit returns where a path fell through to null and
  violated the declared type:
  - StreamRepository::getById/getWatchFolder, GroupService::getById,
    getStream() → return false (declared array|false).
  - ServerRepository::getPublicURL → return '' when server missing (array→string).
  - MagService::resetSTB → return query() result (declared bool).
  - StreamUtils::getPlaylistSegments → explicit return null.
  - NetworkUtils::stopDownload → @return null corrected to @return void.
- PlexController: getPlexToken() called with 5 args but accepts 4 — dropped
  the dead 5th argument.
- DropboxClient::getMetaFromHeaders: array_shift() on an array_filter()
  expression (not a variable, by-ref error) — assign to a var first.
- WatchdogCommand: wrap numeric-string subtractions (nginx/proc-stat values)
  in floatval()/intval().

tools/phpstan/phpstan-bootstrap.php

@@ -25,6 +25,5 @@ if (!defined('MAIN_HOME')) {
     define('MAIN_HOME', $srcRoot . '/');
 }
 
-// Declare constants the app defines at runtime (inside methods) so PHPStan can
-// resolve them. Auto-generated — regenerate with gen-constants-stub.php.
-require __DIR__ . '/constants.stub.php';
+// NOTE: constants.stub.php is loaded via its own bootstrapFiles entry in
+// phpstan.dist.neon (so PHPStan tracks it for result-cache invalidation).