OpenAPI 3.0 request schema generation

[Serbel97]

Closes #72

What

Adds a dependency-free generate_form_schema() helper (new django_api_forms.openapi module, exported from the package root) that converts any Form class into an OpenAPI 3.0 Schema Object describing its JSON request body:

from django_api_forms import generate_form_schema

schema = generate_form_schema(AlbumForm)  # plain dict, ready to mount into an OpenAPI document

How

• Field → schema conversion is dispatched through a class → handler registry resolved along each field's MRO, so custom subclasses inherit their parent's behaviour and unknown field types degrade gracefully to {} (any type) instead of raising
• Django scalars map to proper type/format pairs (date-time, uuid, email, uri, ...) with minLength/maxLength/minimum/maximum constraints where defined
• Library fields are fully covered: FormField recurses into nested objects, FormFieldList/FieldList become arrays with minItems/maxItems, EnumField/ChoiceField become enums, DictionaryField uses additionalProperties, FileField/ImageField become string/byte (Data URI payload)
• required=True fields are collected into the object's required array; label/help_text are exported as title/description
• Meta.mapping is respected, so property names match the JSON keys clients actually send (e.g. _name instead of name)

Scope

Deliberately schema-generation only — no bundled Swagger UI/ReDoc views and no new dependencies. The returned dict plugs into whatever documentation pipeline a project already uses; docs/openapi.md shows how to serve a complete OpenAPI document with Swagger UI/ReDoc from it.

Testing

• 17 new unit tests in tests/test_openapi.py covering every supported field type, nesting, required propagation, Meta.mapping and the unknown-field fallback
• flake8 clean, full suite passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.23%. Comparing base (e27be39) to head (7ab2e6a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   95.09%   96.23%   +1.14%     
==========================================
  Files           9       10       +1     
  Lines         469      585     +116     
==========================================
+ Hits          446      563     +117     
+ Misses         23       22       -1     

Files with missing lines	Coverage Δ
django_api_forms/__init__.py	100.00% <100.00%> (ø)
django_api_forms/fields.py	97.21% <100.00%> (+0.64%)	⬆️
django_api_forms/openapi.py	100.00% <100.00%> (ø)

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