[Feature] Wire fasttext lang detector through maps infrastructure

[Feature] Wire fasttext lang detector through maps infrastructure

The fasttext language detector now supports HTTP/HTTPS URLs for model
loading via the maps system, enabling automatic download, disk caching,
periodic reload, and cross-worker mmap sharing.

Changes:
- fasttext_model::load() accepts an offset parameter for mmap at a
  non-zero position (used with page-aligned map cache files)
- fasttext_langdet uses rspamd_map_is_map() to detect URLs vs local
  paths; URLs go through rspamd_map_add() with RSPAMD_MAP_FILE_NO_READ
- Map callbacks (read/fin/dtor) handle atomic model swap on reload
- Local file paths continue to work as before with direct loading