feat: autocomplete / query suggestions

[arrrnav]

Surface vocabulary terms as the user types.

Approach

• Build a trie from document_freqs keys at startup — no reindexing needed
• Expose a /suggest?prefix=<str> endpoint returning the top N matches
• Hook into the frontend search input with a debounced fetch, showing results in a dropdown

Notes

• Prefix lookup is O(k) where k = query length
• Trie construction happens once at server startup alongside the existing index load