Improve search: multi-term AND + relevance ranking (FTS spike)

[rdhyee]

Summary

Closes #84 — FTS spike complete with immediate search improvements and documented future path.

Shipped now (zero new dependencies):

• Multi-term search: "pottery Cyprus" requires BOTH words to match (was OR on the full phrase)
• Relevance ranking: results sorted by score when searching — label match = 3pts, place = 2pts, description = 1pt
• When not searching, results remain random for exploration variety

FTS spike findings:

• Built offline DuckDB FTS index with tools/build_fts_index.py
• Full index (label + description + place_name): 358 MB — too large for auto-download
• Lite index (label + place_name only): 211 MB — still substantial
• BM25 scoring works well (Porter stemming, English stopwords)
• ATTACH over HTTP in DuckDB-WASM is supported but downloading 200-358 MB is impractical

Recommended next steps (not in this PR):

1. Explore pre-tokenized search parquet (inverted index as parquet, much smaller)
2. Consider on-demand FTS loading behind an "Enhanced Search" toggle
3. Evaluate DuckDB text analytics functions (stemming without full index)

Test plan

• Search "pottery" → results ranked by relevance (label matches first)
• Search "pottery Cyprus" → only samples matching BOTH words
• Search "basalt" → geological samples with label matches at top
• Clear search → results return to random sampling
• Verify tools/build_fts_index.py runs successfully with local parquet

🤖 Generated with Claude Code