Overhaul and modernize Nix flake setup (#706)

* add envrc for direnv users

this was already used by several developers, this just makes it
official

* replace run-tests-script with nix flake check

this also allows us to remove our single 'spago test' invocation
in a github workflow, so all CI checks are in the Nix checks for
Garnix to execute

while i was at it, i removed the nix flakes wrapper, shell.nix,
and flake-compat inclusions as at this point pretty much everyone
is on a flake-compatible version of Nix

* extract wiremock mappings

* migrate to buildNpmPackage from slimlock

slimlock was the homegrown npm dependency builder from ps-overlay
from before when buildNpmPackage was a suitable choice for the
registry, but at this point it's well-suited to be the standard
choice for us.

* overhaul nix flake setup

our homegrown nix setup has diverged quite a bit from modern
best practices and is hard to read. it also included some
unnecessary helpers (like run-vm to work on a local vm) which
we can remove.

this change overhauls the full setup while maintaining the
existing tests to make it simpler, more performant, and more
usable for contributors.

the deployments are still simple ('colmena apply').

Author: thomashoneyman

.envrc

@@ -0,0 +1 @@
+use flake

.github/workflows/main.yml

@@ -19,4 +19,3 @@ result
 
 # Keep it secret, keep it safe.
 .env
-.envrc

.gitignore

@@ -39,9 +39,23 @@ There are three more directories containing code for the registry.
 
 Finally, the `flake.nix` file orchestrates builds for the whole repository.
 
+## Running the Registry Server Locally
+
+The registry server requires a `.env` file and an initialized database. To run the server for development:
+
+```sh
+# 1. Ensure database is initialized (only needed once)
+dbmate up
+
+# 2. Run the server (from the nix shell)
+cd app && spago run
+```
+
+The server will load environment variables from the `.env` file in the project root and run on port 8080 by default.
+
 ## Available Nix Commands
 
-The Registry server can be run locally:
+You can also run the packaged registry server:
 
 ```sh
 nix run .#server
@@ -81,13 +95,6 @@ There is an integration test that will deploy the registry server and make reque
 nix build .#checks.x86_64-linux.integration
 ```
 
-You can "deploy" the registry server to a local VM and manually hit the API as if it were the production server:
-
-```sh
-# The server will be available at localhost:8080
-nix run
-```
-
 ### Testing Guidelines
 
 The PureScript code in the registry is well-tested, ranging from tests for individual functions to full end-to-end tests for the registry server running in a NixOS machine configured the same way as the deployed machine. The smaller and more pure the test, the easier it is to write and maintain; most code is tested via unit tests written with `spec`, and only the core pipelines are run in the integration test.