fix(hts): create the concepts_search_fts FTS5 table it uses (#274)

[mauripunzueta]

Fixes #274.

The bug

crates/hts/src/backends/sqlite/value_set.rs reads from and writes to a concepts_search_fts FTS5 table (the typeahead ranker fts_candidates_ranked_for_system and its populate path populate_concepts_search_fts_for_system), but the table's CREATE VIRTUAL TABLE existed nowhere in the repository:

$ grep -rn "CREATE VIRTUAL TABLE.*concepts_search_fts" .
(no matches)

So the first text-filtered expansion of any system fails hard with no such table: concepts_search_fts, surfaced as an OperationOutcome exception. main currently fails value_set_ops::expand_is_a_filter_with_text_filter_still_nests on exactly this.

Why CI didn't catch it — semantic merge conflict

PR	added	green because
#194	the code that uses the table (no DDL)	nothing in its tests reached the build path
#233	expand_is_a_filter_with_text_filter_still_nests, which does reach it	its CI ran against a base predating #194

Both merged green; the combination on main is broken.

The fix

Add the table next to its siblings in schema.rs, and clear it at startup in the same two blocks that reset concepts_fts / concepts_word_fts.

The shape is derived from how value_set.rs uses it, not guessed:

• term is the only tokenised column — the INSERTs fill it from both the concept's display and every designation value, which is what lets a query match a concept by any synonym and rank with bm25.
• code is retrieved (SELECT code … GROUP BY code) and system_id is a post-filter (WHERE system_id = ?), so both are UNINDEXED.
• Same trigram case_sensitive 0 tokenizer as concepts_fts, its empty-index fallback, so substring matching is consistent between the two.

CREATE VIRTUAL TABLE IF NOT EXISTS concepts_search_fts
USING fts5(system_id UNINDEXED, code UNINDEXED, term,
           tokenize='trigram case_sensitive 0');

Verification

Against a real SQLite (3.45): the DDL applies, both populate INSERTs run, the EXISTS populated-probe returns true, and a trigram MATCH on a designation value returns the owning concept's code ranked by bm25.

@sandhums (author of #194) — please sanity-check the column/tokenizer choice against your intent for the search ranker; the behaviour is inferred from the queries, but you'd know if term was meant to carry anything else.

Found while rebasing #261 (issue #200) onto main; #261's CI is blocked on this because GitHub tests the PR merged into main.

[mauripunzueta]

Superseded by #273, which merged the same fix (create concepts_search_fts FTS5 table + valid ranked query) into main on 2026-07-15. main is green and no longer needs this. Closing as redundant.