CELDEV-1304 - Filebase-Replacement: implement new celements-filebase backend

[fpichler]

https://synjira.atlassian.net/browse/CELDEV-1304

Summary

Adds the backend support for the new MediaLib/VueFinder admin frontend. The branch introduces a celements-filebase backend module with file listing, upload, delete, search, preview/download URLs, and tag management endpoints backed by the central filebase document.

Changes

• Adds VueFinderFilesController under /files for VueFinder-compatible filebase access.
• Supports listing, searching, uploading, and deleting filebase attachments.
• Adds download and preview URLs to file responses.
• Adds filebase tag support:
  • list tags
  • list files for a tag
  • assign/remove tags on files
  • create, rename, and delete tags
  • expose whether the current user may manage tags
• Adds SingleDocFileBaseService implementation for a configured central filebase document.
• Extends filebase service contracts for attachment creation, deletion, matching, and tag operations.
• Adds FileBaseTag model for resolving tag metadata and tagged attachment references.
• Adds tag-specific exception types and an AllAttachmentMatcher.
• Moves/introduces filebase-related backend classes into the celements-filebase module.
• Adds tests for filebase document resolution and VueFinder filename normalization.