fix: use datetime for user soft delete

[xiaoxin-zk]

This fixes a compatibility issue with deployments that share the upstream
ew-api MySQL schema.

Problem:

• users.deleted_at is commonly datetime(3) in
  ew-api

ew_api_tools soft-delete code writes Unix timestamps ( ime.Now().Unix()) into deleted_at

• on MySQL this causes delete-user requests to fail with Incorrect datetime value, and changing the shared column to BIGINT breaks
  ew-api with scan errors like unsupported Scan, storing driver.Value type int64 into type *time.Time

Fix:

• write ime.Time for single-user soft delete
• write ime.Time for batch soft delete
• keep Unix timestamps only for activity threshold calculations
• surface batch update errors instead of ignoring them

Validated on a live deployment:

• upstream
  ew-api kept working with users.deleted_at = datetime(3)
• DELETE /api/users/:id?hard_delete=false returned 200 after the change