Add i18n support for parser/lexer/schema error messages

[mpasternak]

Summary

Adds Django i18n support for every user-facing error message produced by DjangoQL — lexer, parser, schema validator, suggestions API view, and the small `error_message.html` template. Ships translation catalogs for 11 locales.

• All error strings in `lexer.py`, `parser.py`, `schema.py`, `views.py` and `exceptions.py` are wrapped with `gettext_lazy`. Multi-variable messages use named `.format()` placeholders so translators can reorder freely (important for Polish, Russian, Japanese, etc. where word order differs from English).
• `templates/djangoql/error_message.html` now uses `{% trans %}` for the "DjangoQL syntax help" label.
• 11 translation catalogs under `djangoql/locale//LC_MESSAGES/`: `pl` is hand-written native-quality Polish (used as the canary by the i18n tests); `de`, `fr`, `es`, `ru`, `uk`, `pt_BR`, `it`, `nl`, `ja`, `zh_Hans` are auto-translated seeds with `# Auto-translated. Review by a native speaker welcome — open a PR.` markers in their `.po` headers.
• New test module `test_project/core/tests/test_i18n.py` (5 tests) asserts that error messages translate under `override('pl')` and stay English otherwise.
• CI: `sudo apt-get install -y gettext` step added, and `django-admin compilemessages` runs in the `djangoql/` working directory before tests — this also validates every `.po` on every matrix entry.
• `MANIFEST.in` updated to ship `.po` and `.mo` files. README gets a new "Internationalization" section. CHANGES.rst gets an "Unreleased" entry.

If a translation is missing for a string or locale, gettext falls back to the English source — so this is a strictly additive change.

Test plan

• `python test_project/manage.py test core.tests` — 50 tests pass (45 existing + 5 new)
• `flake8 djangoql test_project` — clean
• `isort --diff -c .` — clean
• `msgfmt --check-format` on all 11 `.po` files — clean (named-placeholder and `%`-format integrity verified)
• Manual: `override('pl')` produces "Niedozwolony znak '@'" instead of "Illegal character '@'"
• Manual: `override('pl')` then `str(StrField.value_types_description)` returns "łańcuchami znaków"
• CI green across the full Django × Python matrix (will verify post-push)

Notes for reviewers

• The Polish locale is the only one with native-speaker quality. The other 10 are best-effort auto-translations intended as seeds for community contributions — feel free to merge as-is or wait for native review per locale. Each `.po` is independently replaceable without touching code.
• Version is intentionally left as `0.19.1` and CHANGES.rst uses an `Unreleased` header so the maintainer can assign the actual version on merge.

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