fix(db): add partial index to optimize object lookup queries

[pinglin]

Because

• Slow SQL queries (200-400ms) observed in artifact-backend pod when uploading many files simultaneously
• The query SELECT ... FROM object WHERE uid = '...' AND delete_time IS NULL doesn't have an optimal index
• While uid is the primary key, adding delete_time IS NULL forces PostgreSQL to do a filter step after the index scan
• Under high concurrency, this causes significant performance degradation

This commit

• Adds migration 000058 with a partial index idx_object_uid_not_deleted on object(uid) WHERE delete_time IS NULL
• This allows the query planner to use a direct index lookup without filtering, significantly improving query performance under concurrent load
• Updates TargetSchemaVersion from 57 to 58