nixos/gitea: don't configure the database if createDatabase == false

[uninsane]

fixes fallout from #266270.

a common idiom is to run the git server as user git, instead of gitea, with configuration like this:

services.gitea.user = "git";
services.gitea.database.user = "git";

after #266270, this requires setting services.gitea.database.createDatabase = false (as recommended by the assertion). however, the module then plumbs defaults which no longer make sense into the gitea config causing a failed connection at runtime:

gitea-pre-start: cmd/migrate.go:40:runMigrate() [F] Failed to initialize
    ORM engine: pq: password authentication failed for user "git"

instead, don't default any of the connection settings when createDatabase == false: error at eval time (instead of runtime) if the user hasn't explicitly configured the remaining connection settings.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[Ma27]

For context, there was a discussion in the past on when to declare dependencies (because requires will cause problems when undefined services are listed in there): #237544

If you declare createDatabase = false;, I'd expect that the module leaves the entire task of configuring the database connection to me.

I think the correct fix is to add an assertion here that ensures that the system user is equal to the database user (and thus the PAM authentication used by createDatabase = true; will work again) and ask everybody who doesn't do that (system user == db user == db name) to configure it themselves.

[uninsane]

@Ma27 took me a bit to find something ergonomic, pushed a patch which i think does a good job.

if services.gitea.database.createDatabase == false, then the module just doesn't default any of the services.gitea.database connection settings. then, typical use case looks like:

services.gitea.enable = true;
services.gitea.database.type = "postgres";
services.gitea.database.createDatabase = false;
# if eval'd here, user will see an error about `config.services.gitea.database.{name,user,...}` being referenced but not defined
services.gitea.database.name = "gitea";
services.gitea.database.user = "git";
services.gitea.database.socket = "/run/postgresql";
# then user is expected to manually configure services.postgresql, and define a `requires` from gitea onto postgresql if applicable