Expect a 400, not a 404, for a record that is gone

Running the password tier against a disposable 5.13.50 instance for the
first time failed its last check. The purge itself was fine — the row really
was gone — but the assertion looked for a 404 and the server answers 400.

`Handler.php` maps Laravel's `ModelNotFoundException` to a 400 carrying
`No query results for model [App\Models\X].` and reserves 404 for a route
that does not exist, so a not-found check written against 404 never fires.
The assertion now matches the status together with the message, so a 400
raised for some other reason still fails, and `Error::Api` says so for
callers, who would otherwise write the same dead branch.

Also adds jq to the devshell, which the MSRV recheck in CLAUDE.md pipes
through but which was only ever resolving from the host.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flake.nix

@@ -37,7 +37,10 @@
         msrvToolchain = toolchainFor msrvVersion [ ];
 
         shellFor = toolchain: pkgs.mkShell {
-          packages = [ toolchain pkgs.pkg-config pkgs.openssl ];
+          # jq is for the MSRV recheck in CLAUDE.md, which pipes `cargo
+          # metadata` through it; the rest is the toolchain plus what
+          # openssl-sys needs to link.
+          packages = [ toolchain pkgs.pkg-config pkgs.openssl pkgs.jq ];
           RUST_SRC_PATH = "${toolchain}/lib/rustlib/src/rust/library";
         };
       in