feat(db): add incremental upsert seed script for WordNet vocabulary

Implements packages/db/src/seed.ts — reads all JSON files from
scripts/datafiles/, validates filenames against supported language
codes and POS, and upserts synsets into  and
via onConflictDoNothing. Safe to re-run; produces 0 writes on
a duplicate run.

documentation/spec.md

@@ -205,7 +205,6 @@ term_glosses
   term_id       uuid FK → terms.id
   language_code varchar(10)     -- NOT NULL
   text          text            -- NOT NULL
-  type          varchar(20)     -- CHECK (type IN ('definition', 'example')), NULLABLE
   created_at    timestamptz DEFAULT now()
 
 language_pairs