Great comment, thanks for the questions.
> unless it's a machine at home,
This is, in fact, the intended model.
The problem with any other model, AFAIK, is that someone else has access to your data unless I implement an encrypted live database, like with homomorphic encryption, but even then, I'm sure at some places it would have to be decrypted in memory in places (like, transcoding videos or encoding images, for starters), and the physical owner of the machine will always have access to that.
I just don't think any other way of doing it is really feasible to truly preserve your privacy. I am likely wrong, but if so, I also imagine it's very tedious, nuanced, error-prone, and restrictive.
(Or maybe I'm just totally wrong!)
> - Could index and storage backend be decoupled, so that I can host my photos etc. elsewhere and, in particular, prevent data duplication?
I know this is contentious for some, but part of the point is to duplicate/copy your data into the timeline. It acts as a backup, and it ensures consistency, reliability, and availability.
Apps like PhotoStructure do what you describe -- and do a good job of indexing external content. I just think that's going to be hard to compel in Timelinize.
> Support for importing Polarsteps trips, and for importing Signal backups (e.g. via https://github.com/bepaald/signalbackup-tools ) would be nice!
Agreed! I played with Signal exports for a while but the format changed enough that it was difficult to rely on this as a data source. Especially since it's not just obvious what changes, it's encryption so it's kind of a black box.
That said, anyone is welcome to contribute more data sources. I will even have an import API at some point, so the data sources don't have to be compiled in. Other scripts or programs could push data to Timelinize.
Just to reiterate, one of the main goals of Timelinize is to have your data. It may mean some duplication, but I'm OK with that. Storage is getting cheap enough, and even if it's expensive, it's worth it.