[TASK] Implement log file listing endpoint

Description

Implement GET /v1/maintenance/_logFiles in MaintenanceResource.java. Returns a sorted list of available log file names from the configured log directory, filtered by the configured regex pattern. The returned file names are used as the {fileName} parameter for the existing tail (/v1/logs/{fileName}/_tail) and download (/v1/maintenance/_downloadLog/{fileName}) endpoints.

Acceptance Criteria

• API caller must receive a sorted list of log file names (relative paths, not absolute)
• Files must be read from the directory configured by TAIL_LOG_LOG_FOLDER (default ./dotsecure/logs/)
• Files must be filtered by the regex configured in TAIL_LOG_FILE_REGEX — use default ".*\\.log$|.*\\.out$" to match TailLogResource behavior
• File names must have the base log directory path stripped (return relative names only)
• List must be alphabetically sorted
• Must require backend user with admin access

Priority

None

Additional Context

• Use FileUtil.getAbsolutlePath() to resolve relative log directory path
• Use FileUtil.getFilesByPattern(logFolder, "*.*") to get all files, then filter by regex — same approach as the JSP
• Note: tail_log.jsp defaults TAIL_LOG_FILE_REGEX to ".*" (line 37) but TailLogResource uses ".*\\.log$|.*\\.out$" (line 101). Use the TailLogResource default for consistency with the existing REST tailing endpoint
• JSP legacy: tail_log.jsp lines 36–57

Start in MaintenanceResource.java and compare the existing TailLogResource defaults and the legacy tail_log.jsp lines 36–57; review FileUtil.getAbsolutlePath() and getFilesByPattern(). Done means the admin-only endpoint returns alphabetically sorted, regex-filtered relative log paths from the configured directory, compatible with the existing tail and download endpoints.