feat: honor omit_unused_structs (#74)

[aimestereo]

AI generated change for internal use, that I share as-is.

Fixes #74

The Python plugin ignored omit_unused_structs because Config had no field for it. This ports Go's filterUnusedStructs so unused table and enum types are omitted from models.py.

• Add OmitUnusedStructs to plugin config
• Filter enums/models after queries are built (Go codegen order)
• Python keep-set: bare name + models.Name (no Go NullXxx wrappers)
• Endtoend fixture mirroring sqlc omit_unused_structs testdata
• Bump CI and goldens to sqlc 1.30.0 so version headers match

Query Params/Row structs stay in query.py (Emit: true).

Made with Cursor

[aimestereo]

Closing in favor of a slimmer PR. This branch mixed the feature with a local sqlc 1.28 → 1.30 golden/CI bump that Fenix needs but upstream does not.

Replacement: new PR from feat/omit-unused-structs-upstream (kernel + new fixture only).