fix: add phase filtering to 15 API endpoints to prevent 高中/初中 data mixing

- Supplemental search (semantic + text): add phase field to entries,
  filter by phase in both _search_supplemental_semantic_candidates and
  _search_supplemental_textbook_pages
- Gaokao search path: skip entirely when phase=初中
- Trending: filter search_logs by phase
- Dict/textbook: filter chunks by phase
- Book-pages: infer phase from book_key prefix instead of missing field
- Analytics (heatmap, coverage, concept-breadth, word-freq): add phase
  to all SQL queries on concept_map, keyword_counts, curated_keywords
- Graph (overview, search): add phase to concept_map, cross_subject_map,
  and chunk co-occurrence queries
- Keywords: add 初中 fallback concept list
- Frontend: phase-aware FALLBACK_CONCEPTS for concept carousel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ieduer