Enhance language handling and Turnstile middleware configuration

[ewlarson]

Summary

• Add b1g_language_sm handling across backend indexing/search, strong params, frontend metadata display, field labels, facets, and public schema docs.
• Introduce a language normalization service so language display values are derived consistently from labels, codes, and mixed source formats.
• Add TURNSTILE_ENABLE_LOCAL support and local-context bypass behavior so local development is smoother unless Turnstile is explicitly enabled.
• Allow the API documentation shell, OpenAPI schema, ReDoc, OAuth redirect, and docs branding assets to bypass rate limiting while keeping interactive /api/v1 requests throttled.

Why

This improves language facet/display consistency for issue #177, makes local browser-gate behavior easier to control, and keeps the API docs reachable even when anonymous API quota has been exhausted.

Validation

• python -m pytest backend/tests/middleware/test_rate_limit_middleware.py backend/tests/middleware/test_rate_limit_integration.py backend/tests/test_sanity.py -q
• python -m ruff check backend/app/middleware/rate_limit_middleware.py backend/tests/middleware/test_rate_limit_middleware.py backend/tests/middleware/test_rate_limit_integration.py
• python -m ruff format --check backend/app/middleware/rate_limit_middleware.py backend/tests/middleware/test_rate_limit_middleware.py backend/tests/middleware/test_rate_limit_integration.py

Fixes #177