Fix descending index KeyError and missing FK columns in table definitions

[dav-m85]

• Strip leading '-' from field names in add_meta_indexes before looking up in _forward_fields_map, fixing a KeyError when indexes use descending fields (e.g. fields=["-timestamp"]).

class AuditLog(models.Model):
    timestamp = models.DateTimeField(...)

    class Meta:
        indexes = [
            models.Index(fields=["-timestamp"]),  # crashes django-dbml
        ]

• Emit FK and OneToOneField columns into table.fields before continuing, fixing "ref error: column not found" in dbdiagram.io caused by ref lines referencing columns that were never declared in the table body. This caused "ref error: column not found" when importing the DBML into dbdiagram.io. Fixed by calling build_field_definition and add_field_index for these fields before the continue, matching the pattern used for regular fields.

class Reception(models.Model):
    supplier = models.ForeignKey(Supplier, ...)  # supplier_id never appears in table body

    # Generated DBML would have:
    # ref: purchasing.Reception.supplier_id > catalog.Supplier.id
    # ...but no "supplier_id" column in the Table block → dbdiagram.io ref error

I hope you won't mind me vibecoding this, I'm more a gopher than a pythonist.

Thanks for your software, saved me a lot of time 🙇