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.

packages/db/package.json

@@ -16,5 +16,8 @@
     "@types/pg": "^8.20.0",
     "drizzle-kit": "^0.31.10"
   },
-  "exports": "./src/index.ts"
+  "exports": {
+    ".": "./src/index.ts",
+    "./schema": "./src/db/schema.ts"
+  }
 }