There's no straightforward way to fill same-value translations for non-translatable glossary strings

### Describe the issue

Yesterday, I have added additional strings to the Nontranslatable component. I.e. I have added them to the main (= "source", even though there is no source for TBX) TBX file and pushed it,

Weblate got the webhook, pulled the repo and added the new strings to English (source language) and added empty srtrings to DE, because the file had .de.tbx (but no entries for German).

But the new sourcde strings didn't have empty (or same-value strings for all the other languages.
To get this working, I had to

- Remove the read-only flag from the component
- Bulk-edit to assign the terminology flag to the new strings
- I struggled, because I tried to select by last changed - which did not work!
(it seems that it doesn't get set on add)
- After searching around, landing at the docs, I saw that there's also an 'added' field
(that field is not visible anywhere in the UI...)
- With the terminology flag set, I had to wait, but this time I knew at least that I cannot trust the statistics to see when the terminology flag has caused the missing (empty) translations to be added)
- Then I did bulk-edit again to remove the terminology floag
- kept waiting for a few minutes
(an earlier attempt where I had removed rerminology and added read-only in a single bulk edit did not work
- Then, bulk-edit once again to add the read-only flag
- But the "copy readonly" strings add-on didn't to anyting (saved the component multiple times)
- Only after an hour, the readonly strings had been copied, suddenly. No idea what hung...

Anyway, there should be an easy (or better: fully automatic) way to populate translations with the source string value for non-translatable strings.

(repost from https://github.com/WeblateOrg/weblate/issues/10204#issuecomment-3167839202)

### I already tried

- [x] I've read and searched [the documentation](https://docs.weblate.org/).
- [x] I've searched for similar filed issues in this repository.

Start by reviewing the current terminology flag, read-only component behavior, and the “copy readonly” strings add-on described in the issue, then check the documentation for how added strings and bulk edits are handled. Define the automatic or one-step workflow for populating same-value translations in non-translatable glossary components, including a reliable completion signal.

Written by the indexing model from the issue text.