FR: Allow power search for "no" tag

It seems not possible to find items which have no tag set.

Ideas would be to allow either of these maybe - what is easiest to implement:
- tag="" (This search is accepted but does not return anything)
- !tag:* (with '*' to be expanded to all available tags, so not one of any available or similar)
- exist:tag (would require exist to work with multiple entries)

or similar.

Start by locating the power-search handling and the existing tag-query behavior. Compare the proposed syntaxes for finding documents with no tags, then confirm the intended syntax and expected results before changing anything. Done means the chosen query returns untagged items without breaking existing tag searches.

Written by the indexing model from the issue text.