fix(build): perbaiki referensi paket zesdex-gateway dan sinkronisasi versi nix

- Dockerfile & default.nix: ganti zesdex-backend -> zesdex-gateway (binary zesdex)
- flake.nix & default.nix: versi 1.13.0 -> 1.18.2
- Cargo.lock: sinkronkan versi workspace member (1.18.0 -> 1.18.2)
- perf(search): kurangi alokasi per-query di semantic search (Vec<String> -> Vec<&str>)
- chore(ci): drop cargo build release dari check job, tambah fmt check
- chore: tambah .dockerignore, perbaiki trailing newline .gitignore

.dockerignore

@@ -0,0 +1,26 @@
+# Rust build artifacts
+target/
+
+# VCS
+.git/
+.gitignore
+
+# Local/secret files
+.env
+.env.*
+!.env.example
+
+# Editor
+.idea/
+.vscode/
+*.swp
+
+# Nix
+result
+result-*
+
+# Kilo metadata
+.kilo/
+
+# Docs lessons
+docs/lesson/