fix: serialize M2M fields as lists of PKs in list/get responses

[foogunlana]

Summary

• Fix PydanticSerializationError when listing/getting models with ManyToMany fields
• Convert M2M field values to lists of primary keys instead of leaving them as Django model instances
• Also convert ForeignKey references to PKs instead of strings for consistency

Fixes #76

Test plan

• Verify that models with M2M fields can be listed and retrieved without errors
• Verify that M2M field values appear as lists of integer PKs in responses
• Verify that ForeignKey fields still serialize correctly (now as PKs instead of string representations)
• Verify that models without M2M fields continue to work as before

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