[TASK] Implement search and replace endpoint

Description

Implement POST /v1/maintenance/_searchAndReplace in MaintenanceResource.java. Performs a database-wide find/replace across text content in contentlet, container, template, field, and links tables. This is a dangerous, irreversible operation — only working/live versions are affected.

Acceptance Criteria

• API caller must provide searchString (non-empty) and replaceString (non-null) in JSON body
• Endpoint must call MaintenanceUtil.DBSearchAndReplace(searchString, replaceString)
• Endpoint must flush all caches and reset permission references after replacement
• API caller must receive a response indicating success or whether errors occurred during replacement
• Must require CMS Administrator role and Maintenance portlet access
• API caller must receive 400 Bad Request if searchString is empty

Priority

None

Additional Context

• MaintenanceUtil.DBSearchAndReplace() uses parameterized SQL REPLACE() calls — no SQL injection risk
• Tables affected: contentlet (text1-25, text_area1-25), containers (code, pre_loop, post_loop), template (body), field (field_values), links (url)
• Only updates working/live versions via version_info joins
• Struts legacy: ViewCMSMaintenanceAction.searchAndReplace() lines 404–440

Start in MaintenanceResource.java and compare the legacy behavior in ViewCMSMaintenanceAction.searchAndReplace() at lines 404–440. Trace MaintenanceUtil.DBSearchAndReplace() and the existing maintenance security, cache, and permission handling. Done means the endpoint meets every listed validation, authorization, replacement, cache-reset, and response acceptance criterion.