feat(config): integrate etcd for service discovery and config management

- Add etcd-client dependency for distributed configuration storage
- Implement EtcdConfig with priority: etcd > environment variables > defaults
- Add ServiceRegistry for service registration with lease keep-alive
- Integrate etcd-based service discovery for appks gRPC connections
- Add service watcher for real-time service instance updates
- Migrate Redis configuration from single URL to cluster node list
- Update Dockerfile with default IMKS_HOST and IMKS_PORT environment variables
- Add etcd bootstrap configuration through environment variables
- Implement Redis cluster URL building with optional authentication

Dockerfile

@@ -18,7 +18,6 @@ RUN cargo build --release --bin imks && \
     strip target/release/imks
 
 FROM ubuntu:26.04
-
 RUN apt-get update && \
     apt-get install -y --no-install-recommends ca-certificates curl && \
     rm -rf /var/lib/apt/lists/*
@@ -27,10 +26,12 @@ COPY --from=builder /app/target/release/imks /usr/local/bin/imks
 COPY --from=builder /app/migrate/ /app/migrate/
 
 WORKDIR /app
-
 RUN useradd -m -u 1000 imks && chown -R imks:imks /app
 USER imks
 
+ENV IMKS_HOST=0.0.0.0
+ENV IMKS_PORT=3000
+
 EXPOSE 3000
 
 HEALTHCHECK --interval=15s --timeout=3s --start-period=10s --retries=3 \