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>

package.json

@@ -40,7 +40,7 @@
     "@inquirer/password": "^1.1.2",
     "@inquirer/prompts": "^1.2.3",
     "@nostr-dev-kit/ndk": "3.0.3",
-    "@prisma/client": "^5.4.1",
+    "@prisma/client": "^6.19.0",
     "@scure/base": "^1.1.1",
     "@types/yargs": "^17.0.24",
     "axios": "^1.6.2",
@@ -65,7 +65,7 @@
   "devDependencies": {
     "@types/debug": "^4.1.8",
     "@types/node": "^18.16.18",
-    "prisma": "^5.4.1",
+    "prisma": "^6.19.0",
     "ts-node": "^10.9.1",
     "tsup": "^7.2.0",
     "typescript": "^5.1.3"