build(backend): one shared Dockerfile with cargo cache mounts, sequential compose build, stripped release binaries

All four services now share one builder stage (dependencies compile once per
deploy, not once per service). BuildKit cache mounts keep the cargo registry
and target/ between deploys: a one-line change rebuilds in ~16 s instead of
recompiling the whole dependency tree.

backend/chat-service/Dockerfile

@@ -1,20 +0,0 @@
-# Build context must be backend/ (the workspace root).
-FROM rust:1-trixie AS builder
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    protobuf-compiler pkg-config libssl-dev ca-certificates \
-    && rm -rf /var/lib/apt/lists/*
-WORKDIR /build
-COPY Cargo.toml Cargo.lock ./
-COPY common ./common
-COPY accounts-service ./accounts-service
-COPY gateway ./gateway
-COPY configs-service ./configs-service
-COPY chat-service ./chat-service
-RUN cargo build --release -p chat-service
-
-FROM debian:trixie-slim
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    ca-certificates libssl3 \
-    && rm -rf /var/lib/apt/lists/*
-COPY --from=builder /build/target/release/chat-service /usr/local/bin/chat-service
-ENTRYPOINT ["/usr/local/bin/chat-service"]