add native Dockerfile for backend (replacing nixpacks)

backend/Dockerfile

@@ -0,0 +1,18 @@
+FROM rust:1-slim-bookworm AS builder
+WORKDIR /app
+RUN apt-get update && apt-get install -y --no-install-recommends build-essential pkg-config && rm -rf /var/lib/apt/lists/*
+
+COPY Cargo.toml Cargo.lock ./
+RUN mkdir src && echo "fn main() {}" > src/main.rs \
+    && cargo build --release \
+    && rm -rf src
+
+COPY src ./src
+RUN touch src/main.rs && cargo build --release
+
+FROM debian:bookworm-slim
+RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates && rm -rf /var/lib/apt/lists/*
+WORKDIR /app
+COPY --from=builder /app/target/release/tssbot-backend ./tssbot-backend
+EXPOSE 6000
+CMD ["./tssbot-backend"]