feat(pages): semantic + full-text search for questions page

[gregagi]

Summary

• add hybrid search to the public questions page (/questions?q=...)
• mix lexical (title/body full-text) and semantic (Qdrant vector) relevance into a combined score
• keep robust fallback behavior: if semantic search is unavailable, use lexical-only ranking
• retain query param through pagination links and expose search_query in view context
• add reusable semantic question search helper in vector indexing module
  • tries Qdrant /points/query first, falls back to /points/search for compatibility
• add pages tests for query context, hybrid ordering (mocked semantic hits), and template pagination query preservation
• update changelog

Testing

• DJANGO_READ_DOT_ENV=0 ENVIRONMENT=test SECRET_KEY=test DEBUG=1 SITE_URL=http://testserver uv run pytest apps/pages/tests.py -q
• DJANGO_READ_DOT_ENV=0 ENVIRONMENT=test SECRET_KEY=test DEBUG=1 SITE_URL=http://testserver uv run pytest apps/api/tests.py -q
• DJANGO_READ_DOT_ENV=0 ENVIRONMENT=test SECRET_KEY=test DEBUG=1 SITE_URL=http://testserver uv run ruff check apps/api/vector_indexing.py apps/pages/views.py apps/pages/tests.py