Lemmy 0.19.12, NixOS 25.05, Postgres 16

we're finally on Lemmy 0.19.12! check out the changes here:

• our infrastructure deployment flake
• the frontend
• the backend

here's a quick summary of what changed:

• our instance fork is now in line with the stable upstream version of Lemmy, 0.19.12. big shoutout to @froztbyte@awful.systems for their significant labor in documenting the upstream changes we could expect from 0.19.3 to the new version, in figuring out what the stable version of Lemmy even is (the 0.20.0 and 1.0.0 series of releases don't even talk to their own frontends), and to froztbyte, @fasterandworse@awful.systems, and @dgerard@awful.systems for moral support during the upgrade process.
• all our instance features merged fine into the new version (this, shockingly, wasn't the hard part).
• our Lemmy Nix module configuration has been moved out of the infrastructure repo, flakeified, and brought in line with the current state of the Lemmy NixOS module. in the process, I fixed two major bugs in the Lemmy NixOS module around secret handling and federation. I will not be upstreaming these changes because the Nix people like murderbots and fascists more than they like having contributors.
• we're now running on the latest stable NixOS, 25.05.
• I've removed the infrastructure code for the now-unused staging instance; now we just have prod and dev.
• we've migrated to PostgreSQL 16, the version currently in use by the Lemmy Docker container.

as always, post here or in the testing thread if anything seems extremely broken