perf(files): keep systemtag and filecache joins index-friendly

[solracsf]

Summary

The system tag search joins compared filecache.fileid (bigint) against systemtag_object_mapping.objectid (varchar). On MySQL/MariaDB this implicitly coerces the objectid string column to a number, which makes any index on objectid unusable and turns the join into a full/BNL scan of the mapping table.

Comparing as strings instead (objectid = CAST(fileid AS CHAR)) keeps the index usable. On a 100k-row mapping table the systemtagmap access dropped from ~50k examined rows per outer row to 1, with identical results (objecttype is filtered to 'files', where objectid is always the decimal fileid). Applied to both the search query and the tag-usage count query.

Also, strip a lone to_char() string cast when parsing partitioned join conditions (the previous branch duplicated the int-cast check and was dead code) so sharded setups extract the join columns correctly on Oracle.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI