fix: rename utils.py → util.py across modules, fix add --index-only, misc cleanups

Renames (no functional change, just consistency with the rest of the codebase):
- cli/cli_utils.py → cli/cli_util.py
- core/host_utils.py → core/host_util.py
- core/tag_utils.py → core/tag_util.py
- crawls/schedule_utils.py → crawls/schedule_util.py
- machine/env_utils.py → machine/env_util.py

Functional fixes:
- archivebox add --index-only now materializes Snapshot rows synchronously
  via crawl.create_snapshots_from_urls() instead of just queueing the Crawl
  and leaving the index empty. The previous behavior broke every test that
  expected --index-only to populate the index, since the runner is never
  started in index-only mode.
- config/collection.py: add _coerce_from_str_dict as the inverse of
  _coerce_to_str_dict so JSON-encoded INI values are decoded back to native
  dict/list types when mirrored into Machine.config (a JSONField). Without
  this, downstream consumers like MachineEvent / abx-dl get raw JSON
  strings where they expect dicts.

Plus matching admin / middleware / model touch-ups, the registration
password_change_form template, and assorted small cleanups the user
worked through while validating the deploy path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>