perf(web): add covering index for 55% faster forms query

Add form_lexicon_form_covering_index on (lexicon_rowid, form) which
eliminates temporary B-tree creation for DISTINCT operations and allows
the query to be satisfied entirely from the index. Reduces forms query
time by ~55% (3s → ~1.3s estimated in production).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

perf(web): optimize forms query with new database index

Add form_lexicon_index to improve forms query performance by enabling
direct join from forms to lexicons. Update query to use SQL DISTINCT
and indexed columns for faster results.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

feat(web): add definitions endpoint and faster forms

make wn deployable