The longer answer is that the browser can talk directly to SAFENetwork, providing storage/messaging features accessible via javascript. This provides some of the functionality that a back end server would traditionally provide.
In a future version of SAFENetwork, compute will be offered, perhaps allowing scripts to be run by the network on behalf of the user too. This would allow more powerful apps to be created.
SAFENetwork requires a different approach to writing web apps. It is more powerful in some ways and less so in others. The latter will likely be improved in subsequent SAFENetwork versions.
It’s possible. There’s a peripheral called Decorum that is deferring a forum on SAFE.
As an example of a CRUD interface on SAFE see the talk and links to info on single page blogging app, talking to SAFE as if it offered an LDP storage service:
It’s not incentivizing long term storage in any way. It’s more built around cache, clearing the cache, and pinning. Pinning being the ONE source that always has the file somewhere (assume it’s you as the creator). When someone asks for that file, the request will make it’s way back to you if there’s no intermediate node with it cached. As it passes back, intermediate nodes may then decide to keep it in cache. That cache will eventually expire unless one of those nodes also decides to pin it and keep it long term. The more a file is asked for, the more it’s cached all over, however if those requests stop, it will eventually be purged unless someone, somewhere has it pinned.
Hence the original materials (predating filecoin I believe) talk about a MARKET to pay someone to pin on your behalf. Filecoin comes along as the layer to automate that marketplace. Meaning in practice you pay a price to have someone ‘store’ (pin) your files. So again, IPFS is really built around a cache layer, and less so long term personal storage.
Proof of spacetime is used to ensure that miners hold the data for some period of time, but you’re right, it isn’t really clear what that period of time would be and if it would be on a contract by contact basis or always the same. You could at least continually renew the contract for long term storage.
Another difference from SAFE is that there is a retrieval market too, so getting data is not free under Filecoin. It is free in IPFS, but then there’s no availability guarantee.