Parser: django-mptt models are skipped entirely (MPTTModel base + TreeForeignKey)

[FROWNINGdev]

Context

Models built on django-mptt are currently invisible to the parser — not merely missing a field, but skipped entirely.

_looks_like_model (cli/django_orm_lens/parser.py:358) decides whether a class is a Django model from its base classes. It accepts models.Model, a handful of common abstract names, anything matching Abstract[A-Z], and anything ending in Mixin. MPTTModel matches none of them, so:

class Category(MPTTModel):          # skipped — never appears in the sidebar,
    name = models.CharField(max_length=50)   # ER diagram, or any analyzer
    parent = TreeForeignKey('self', on_delete=models.CASCADE, null=True)

The second half of the problem is TreeForeignKey / TreeOneToOneField / TreeManyToManyField. These are thin subclasses of the corresponding Django fields, but the parser matches field types against the literal whitelist in BARE_FIELD_TYPES (parser.py:59) and relations against RELATION_TYPES (parser.py:38), so even inside a detected model they would be dropped.

What to change

1. Teach _looks_like_model about MPTTModel (and MPTTMeta-style bases if you find others worth adding).
2. Add the three Tree* field names to BARE_FIELD_TYPES and to RELATION_TYPES, so _extract_related / _extract_on_delete / _extract_related_name (lines 221–278) treat them exactly like their Django counterparts. TreeForeignKey('self', ...) must produce the same self-referential edge that a plain ForeignKey('self', ...) does — there is already a test for the self-FK case at cli/tests/test_er_formats.py:112.

Please keep this additive: no django-mptt import, no new dependency. The parser is deliberately pure-static and must keep working on a project whose venv is broken.

Acceptance criteria

• A model inheriting MPTTModel shows up in django-orm-lens scan with its fields.
• TreeForeignKey('self', ...) renders a self-edge in er, honouring on_delete and related_name.
• A fixture under cli/tests/fixtures/ covering both, wired into the test suite.

Getting started

pip install -e "cli[dev]"
pytest cli/tests -v -k parser

Note this is one half of the "third-party field support" roadmap item; django-taggit is tracked separately in #50 — feel free to take either or both.