feat(config): integrate etcd-based configuration management

- Add etcd-client dependency with TLS support
- Implement EtcdConfig struct for reading config values with priority: etcd > env > default
- Add ServiceRegistry for service discovery registration in etcd
- Create from_etcd method in AppConfig for loading SMTP configuration
- Update main.rs to use etcd-based config loading with fallback mechanism
- Add etcd module with client connection and key-value operations
- Modify Dockerfile to use cargo-chef for faster builds
- Add docker-compose.yaml for emailks service deployment
- Include AGENTS.md with development guidelines and best practices
- Add build.sh script for podman-based container building
- Update dependencies in Cargo.toml and Cargo.lock

Dockerfile

@@ -1,38 +1,33 @@
-# ---- builder ----
-FROM rust:1.96-slim-bookworm AS builder
-
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    pkg-config \
-    libssl-dev \
-    protobuf-compiler \
-    && rm -rf /var/lib/apt/lists/*
-
+FROM rust:1.96-bookworm AS chef
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        protobuf-compiler libprotobuf-dev \
+        pkg-config libssl-dev \
+        mold clang && \
+    rm -rf /var/lib/apt/lists/*
+RUN cargo install cargo-chef
 WORKDIR /app
 
-# Cache dependencies
-COPY Cargo.toml Cargo.lock ./
-COPY build.rs ./
-COPY proto/ proto/
-RUN echo '' >lib.rs && \
-    echo 'fn main() {}' >main.rs && \
-    cargo build --release --bin emailks; \
-    rm -f lib.rs main.rs
+FROM chef AS planner
+COPY . .
+RUN cargo chef prepare --recipe-path recipe.json
 
-# Build real binary
+FROM chef AS builder
+COPY --from=planner /app/recipe.json recipe.json
+RUN cargo chef cook --release --recipe-path recipe.json
 COPY . .
 RUN cargo build --release --bin emailks && \
-    cp target/release/emailks /app/emailks
+    strip target/release/emailks
 
-# ---- runtime ----
-FROM debian:bookworm-slim
+FROM ubuntu:26.04
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        ca-certificates libssl3 && \
+    rm -rf /var/lib/apt/lists/*
+COPY --from=builder /app/target/release/emailks /usr/local/bin/emailks
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    ca-certificates \
-    libssl3 \
-    && rm -rf /var/lib/apt/lists/*
-
-COPY --from=builder /app/emailks /usr/local/bin/emailks
+ENV EMAILKS_HOST=0.0.0.0
+ENV EMAILKS_PORT=50051
 
 EXPOSE 50051
-
 ENTRYPOINT ["emailks"]