Your web server can easily host SQLite. It’s the only mainstream database that doesn’t need a server. It’s actually just a single file of C code, and most languages have an api binding to it so you don’t have to set anything up besides giving it a file to write to!

Backup, yes you can just copy the file to back it up, but SQLite documentation has a preferred method that uses its own backup api which is likely safer than a file copy.

Replication, I don’t think there is a way to keep multiple DB’s in sync. You will likely need to move to a client server DB like Postgres if you get to that point.

Availability, only allows one writer at a time, so that’s the biggest issue for performance. The docs say it’s used on websites that get 100k’s or hits a day, but likely not great for workloads that have a high amount of writes. You obviously will be constrained by how fast the servers CPU and I/O is.

The author of SQLite says for blobs of about 20kb or less, it’s faster than reading and writing to the file system due to the overhead of opening a file.