Propagate translations into newly added strings

### Describe the problem

As discussed in https://github.com/orgs/WeblateOrg/discussions/14705.

Apparently propagation only happens when actually translating.
If a new string appears in a component that already has a translation elsewhere, the new string will not get the translation.

### Solution brainstorm

I think that expectation is that new strings will get a propagated translation immediately.

Another alternative could be a delayed solution (daily task?).

Could be gated with a setting for people who don't want this for some reason.

### Describe alternatives you have considered

I have tried "Automatic translation > Source: Other translation components".
It does the job. But it has be to run per each language, and takes quite some time. And of course, manual interventions are always error prone.

### Screenshots

_No response_

### Additional context

_No response_