[US-2.4 / OBT-264] Reactivate a deactivated language

[levigtri]

Summary

US-2.1 / #97 made language deletion a soft-delete (is_active = False) gated behind require_platform_admin, but left no way back. This adds the inverse operation, mirroring deactivate_language exactly.

• Service reactivate_language (app/services/language/reactivate_language.py): platform-admin-only, sets is_active = True, get_language_or_404 → NotFoundError, AuthorizationError for non-admins. Idempotent (never inspects the current flag, same as deactivate).
• Endpoint POST /api/languages/{language_id}/reactivate → 200 with LanguageResponse, guarded by require_platform_admin.
• Exported on language_service.

No Alembic migration — Language.is_active already exists on the base branch (introduced by #97).

Stacked on #97 — merge order

This branch is stacked on levigft/obt-200-us-21-... (#97), because reactivate_language mirrors deactivate_language and reuses Language.is_active, both of which are introduced there. Same arrangement as #90.

Merge #97 before this PR. GitHub retargets this PR to main automatically once #97 lands.

Test plan

uv run pytest tests/test_language_service.py tests/test_language_api.py

• Service (tests/test_language_service.py): admin reactivates an inactive language → active; non-admin → 403; missing → 404; reactivated row reappears in the default list_languages; reactivating an already-active language is a no-op.
• API (tests/test_language_api.py, new): admin POST …/reactivate → 200 with is_active: true; authenticated non-admin → 403 (exercises the require_platform_admin dependency, not just the service guard); missing id → 404.

All green (ruff + mypy clean; tests/ is excluded from mypy per pyproject.toml, matching the existing suite).

Frontend counterpart

The console reactivate affordance (a restore action on inactive rows, platform-admin only) plus the darker inactive-row styling ship on the redesign line — tripod-console#40 (OBT-258).

Closes OBT-264.

[linear-code]

OBT-264

[coderabbitai]

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