chore: replace Docker setup with Nixpacks configuration

- Remove Dockerfile and docker-compose.yml in favor of nixpacks.toml
- Keep backup files (Dockerfile.bak, docker-compose.yml.bak) for reference
- Nixpacks provides a simpler, declarative approach to building and deploying

nixpacks.toml

@@ -0,0 +1,13 @@
+providers = ["node"]
+
+[phases.setup]
+nixPkgs = ["python3", "gnumake", "gcc", "sqlite"]
+
+[phases.install]
+cmds = ["npm ci"]
+
+[phases.build]
+cmds = ["npm run build"]
+
+[start]
+cmd = "npm run start"