feat(api): add OEM breakdown endpoint and group chart

[Harsh63870]

Summary

Fixes #1569.

Nebraska already stores instance OEM, and the UI already has a version distribution pattern. This adds the OEM equivalent end to end:

• DB/API: GetGroupOEMBreakdown (empty OEM → unknown, same 1-day activity window)
• OpenAPI: GET /api/apps/{app}/groups/{group}/oem_breakdown + regenerated codegen
• Frontend: OEM/platform breakdown on the group card (reuses the version breakdown bar)

Test plan

• cd backend && go test ./pkg/api/ -run TestGetGroupOEMBreakdown
• Hit GET /api/apps/<app>/groups/<group>/oem_breakdown and confirm {oem,instances,percentage}
• Open an applications list / group card and confirm the OEM chart renders beside version breakdown

Related

• OEM distribution is recorded but never exposed in metrics or the dashboard #1470 (Prometheus OEM gauge)
• instance oem and aleph_version are served but undeclared, and the list endpoint omits them #1454 (oem fields on instance JSON)

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