docs(db): stop calling a live query an unimported example (#305)

The paragraph describing buildItemFilterSql claimed "nothing imports it" but it is actually a live production function defined at src/itemFilters.ts:264 and imported by both src/routes/admin.ts and src/routes/items.ts. The conversion example in the conventions file was mistakenly described as though it were the function itself rather than as an example demonstrating the query pattern. Fixed the wording to clarify that the function remains raw pg code and the shown conversion is an example of how to convert it, not a committed version in src/ waiting to be called.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/db-kysely/CONVENTIONS.md

@@ -37,7 +37,7 @@ Kysely uses the `pg` driver directly, so a Postgres SQLSTATE stays on `err.code`
 
 ## The worked example
 
-`buildItemFilterSql` is the hardest query in the codebase — six optional clauses composed at run time, a recursive CTE for the category subtree, an `ANY(...::int[])` tag match with a count equality, and array parameters. It was the #216 spike's target and #297's, and it is here rather than in a source file because nothing imports it:
+`buildItemFilterSql` is the hardest query in the codebase — six optional clauses composed at run time, a recursive CTE for the category subtree, an `ANY(...::int[])` tag match with a count equality, and array parameters. It was the #216 spike's target and #297's, and it is still raw `pg` in `src/itemFilters.ts`, used by both the storefront and the admin listing. The conversion below is an example rather than a source file: committing it as one would put a second copy of a live function in `src/` that nothing calls, which is precisely what the #216 spike became.
 
 ```ts
 if (filters.categoryIds.length) {