fix: tag search never matches on slug name

[ngineer420]

Proposed Changes

Tag search never matches on slug_name.

In tagCommonRepo.GetTagPage, the search term is formatted into LOWER(%s) and
passed as the value of the LIKE, so the function name ends up inside the
pattern rather than being applied to the column:

builder.Like{"slug_name", fmt.Sprintf("LOWER(%s)", tag.SlugName)}
// -> slug_name LIKE '%LOWER(coco)%'

That can never match. Only the display_name clause does any work, and LIKE
is case-sensitive on PostgreSQL, so searching for a tag by the name it is
written in returns nothing:

GET /answer/api/v1/tags/page?slug_name=Coco   ->  ["coco"]
GET /answer/api/v1/tags/page?slug_name=coco   ->  []

Tags are lower case by convention, so lower case is what people type, and the
tag filter appears to report that no such tag exists.

• Lower both the column and the term so the comparison is case-insensitive
• Extract the term normalisation into searchTermForTag so the behaviour can
  be covered by a unit test without standing up a database
• Add that test

Reproduced against 2.0.2 on PostgreSQL 18.