deps: bump prisma 5.4.1 → 6.19.3 (nix build fix)

Required to keep the nix package buildable: nixpkgs unstable no longer
ships prisma-engines 5.x — the unsuffixed `prisma-engines` attr now
aliases 7.x (no libquery_engine.node), and the only versioned attrs are
`prisma-engines_6` (6.19.3) and `prisma-engines_7`. Bump both
`@prisma/client` and `prisma` to ^6.19.0 so the client matches the only
engine we can pin to.

Also:
- package.nix takes `prisma-engines_6` directly. flake.nix passes
  `pkgs.prisma-engines_6 or pkgs.prisma-engines` so the package still
  builds on nixos-25.05 (where prisma-engines is 6.7.0 unsuffixed).
- Drop PRISMA_INTROSPECTION_ENGINE_BINARY — prisma 6 collapsed the
  introspection engine into schema-engine, the binary no longer ships.

Schema is unchanged so existing fresh installs migrate identically.
Existing dev instances with a prisma_5-tracked _prisma_migrations table
will need a one-time `prisma migrate resolve` step on first boot under
the new client; deploy targets are all fresh installs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flake.nix

@@ -14,7 +14,13 @@
         let pkgs = pkgsFor system; in
         rec {
           default = nsecbunkerd;
-          nsecbunkerd = pkgs.callPackage ./package.nix { };
+          nsecbunkerd = pkgs.callPackage ./package.nix {
+            # nixos-unstable splits prisma-engines into versioned attrs and
+            # aliases the bare `prisma-engines` to 7.x (no libquery_engine
+            # for our 6.x client). nixos-25.05 has only the bare attr at
+            # 6.7.0. Pick whichever exists so the package builds on both.
+            prisma-engines_6 = pkgs.prisma-engines_6 or pkgs.prisma-engines;
+          };
         }
       );