fix(organizations): add REST endpoint for branding logo upload signing

[arthurzeras]

Summary

• The Organization Auth-Area Branding feature's logo upload always 403'd from the frontend SPA: the shipped django-s3direct signing view (POST /s3direct/get_upload_params/) is a plain Django view authenticated only by session cookie, never reachable by DRF's JWTAuthentication — so the JWT-only SPA always hit it as AnonymousUser.
• Adds OrganizationBrandingLogoUploadParamsView (POST /branding/logo-upload-params/), the REST sibling of the existing GraphQL create_branding_logo_upload mutation, reusing organizations.branding_logo.sign_branding_logo_upload for the actual signing.
• Gated on the two-condition branding eligibility check (parentless + entitled), not the three-condition write gate — the SPA uploads a logo on file-picker change, before the slug/branding PUT on form submit. Factored the shared eligibility check into organizations.permissions.check_branding_read_eligibility, reused by both this view and OrganizationBrandingView.get.
• Regenerated schema.yml.

See .vinta-ai-workflows/handoffs/2026-08-06-branding-logo-upload-403.md and ai-plans/2026-08-04-ORGANIZATION_AUTH_BRANDING_SPEC.md / ai-plans/2026-08-04-ORGANIZATION_AUTH_BRANDING_IMPLEMENTATION_PLAN.md for full context.

Test plan

• New tests in organizations/tests/test_branding_logo_upload_params.py: happy path, no-slug-org still admitted, non-admin/parented-org/unentitled-org 403s, unauthenticated 401, bad content-type/oversized-file 400s, multi-org-admin-without-header 400 (proves TenantScopedViewMixin is wired).
• ruff check / ruff format --check clean.
• mypy clean on touched files.
• makemigrations --check — no changes needed (no model change).
• Full test suite green on a fresh database (4869 passed).
• Manual check against a running stack with a real JWT (left to reviewer/QA, or the frontend agent once wired up).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.42%. Comparing base (f338491) to head (07e6c68).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
organizations/views.py	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #231      +/-   ##
==========================================
+ Coverage   87.41%   87.42%   +0.01%     
==========================================
  Files         254      254              
  Lines       21735    21769      +34     
  Branches     2446     2447       +1     
==========================================
+ Hits        19000    19032      +32     
- Misses       1944     1945       +1     
- Partials      791      792       +1     

Flag	Coverage Δ
unittests	87.42% <94.87%> (+0.01%)	⬆️

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