D2.1 Mount the Studio shell and re-home DTC Studio pages

Plan issue D2.1 (community-base plan phase 2, docs/plan/phase-2.md). Depends on C2.4 (package release v0.3.0, done).

Goal: mount community_base.studio as the Studio shell and move the DTC Studio pages under it.

Scope:

• Install community_base.studio (v0.3.0 is already pinned; no package change needed).
• Delete templates/studio/base.html; DTC Studio templates extend community_base/studio/base.html.
• Keep DTC's authorize_studio_request roles by implementing the kernel hook STUDIO_AUTHORIZER (default: is_staff).
• Register sections: Site (settings, navigation, sponsors), Access (API keys from the package, credentials until management_api migrates), Audit, Events (identities, historical totals, Q&A), Courses (all studio_courses pages).
• Remove the Studio adapter half of management_registry.py; management_api keeps its routes until its endpoints are re-declared through community_base.api in later phases.
• Exempt Studio routes from the public-page inline-stylesheet test; keep the test for public pages.

Non-goals:

• No management_api endpoint changes.
• No public-page shell changes.

Verification:

• uv run python manage.py studio_routes --check -> OK.
• uv run pytest studio studio_courses -q -> pass.
• Accessibility Playwright markers for Studio pass.

Read first: templates/studio/base.html, studio/views.py, studio/urls.py, studio_courses/, management_registry.py, accounts/studio_authorization.py, accounts/studio_roles.py.

Read templates/studio/base.html, studio/views.py, studio/urls.py, studio_courses/, management_registry.py, accounts/studio_authorization.py, and accounts/studio_roles.py to map the current shell, routes, sections, and authorization hook. Verify with uv run python manage.py studio_routes --check, uv run pytest studio studio_courses -q, and the Studio accessibility Playwright markers; done means the Studio shell and listed sections work without changing public pages or management_api endpoints.