fix: drop cross-compilation, build natively in Dockerfile

podman-compose does not populate BUILDPLATFORM/TARGETARCH build args, so
the platform-detection logic always fell back to x86_64 — even on arm64.
This caused cc-rs to look for 'x86_64-linux-gnu-gcc' instead of 'gcc'.

Replace the entire cross-compile scaffolding with a plain native build:
  cargo build --release (no --target)

Cargo targets the host platform automatically. If cross-compilation is
ever needed it can be reintroduced with a properly-tested setup.

https://claude.ai/code/session_01FKCW3FDjNFj6jve4niMFXH

infra/Dockerfile.server

@@ -1,70 +1,28 @@
 # syntax=docker/dockerfile:1
 # ── Build stage ───────────────────────────────────────────────────────────────
-# Run the compiler on the *build* host; cross-compile to target when needed.
-FROM --platform=$BUILDPLATFORM docker.io/library/rust:1.94-slim-bookworm AS builder
+# Native build: Cargo targets the host platform automatically.
+# No --target flag means no cross-compiler confusion regardless of which
+# arch podman-compose runs on (x86_64, arm64, armv7…).
+FROM docker.io/library/rust:1.94-slim-bookworm AS builder
 
-ARG BUILDPLATFORM
-ARG TARGETPLATFORM
-ARG TARGETARCH
-ARG TARGETVARIANT
-
-# Install build tools. gcc is always needed: cc-rs (used by ring/aes-gcm deps)
-# calls the native compiler even on native builds, and rust:slim doesn't
-# include it.  Cross-compilers are added only when actually cross-compiling.
+# gcc is required by cc-rs (used by aes-gcm / ring build scripts).
+# rust:slim does not include a C compiler.
 RUN apt-get update && apt-get install -y gcc pkg-config && \
-    if [ "${BUILDPLATFORM}" != "${TARGETPLATFORM}" ]; then \
-      case "${TARGETARCH}:${TARGETVARIANT}" in \
-        "arm64:")  apt-get install -y gcc-aarch64-linux-gnu    ;; \
-        "arm:v7")  apt-get install -y gcc-arm-linux-gnueabihf  ;; \
-        "arm:v6")  apt-get install -y gcc-arm-linux-gnueabi    ;; \
-      esac; \
-    fi && \
     rm -rf /var/lib/apt/lists/*
 
-# Map TARGETARCH + TARGETVARIANT → Rust target triple, then install it.
-RUN case "${TARGETARCH}:${TARGETVARIANT}" in \
-      "amd64:")  echo x86_64-unknown-linux-gnu         ;; \
-      "arm64:")  echo aarch64-unknown-linux-gnu         ;; \
-      "arm:v7")  echo armv7-unknown-linux-gnueabihf     ;; \
-      "arm:v6")  echo arm-unknown-linux-gnueabi         ;; \
-      *)         echo x86_64-unknown-linux-gnu          ;; \
-    esac > /rust_target && \
-    rustup target add "$(cat /rust_target)"
-
-# Tell Cargo which linker to use for each target.
-# x86_64 native: use plain gcc (cc-rs would otherwise look for
-# "x86_64-linux-gnu-gcc" which is the *cross* toolchain, not installed here).
-RUN mkdir -p /root/.cargo && printf '\
-[target.x86_64-unknown-linux-gnu]\n\
-linker = "gcc"\n\
-\n\
-[target.aarch64-unknown-linux-gnu]\n\
-linker = "aarch64-linux-gnu-gcc"\n\
-\n\
-[target.armv7-unknown-linux-gnueabihf]\n\
-linker = "arm-linux-gnueabihf-gcc"\n\
-\n\
-[target.arm-unknown-linux-gnueabi]\n\
-linker = "arm-linux-gnueabi-gcc"\n' >> /root/.cargo/config.toml
-
 WORKDIR /build
 
-# Cache dependencies separately from source.
+# Cache dependency compilation separately from application source.
 COPY Cargo.toml Cargo.lock* ./
 COPY server/Cargo.toml ./server/
 RUN mkdir -p server/src && echo 'fn main(){}' > server/src/main.rs
-RUN TARGET=$(cat /rust_target) && \
-    cargo build --release --target "$TARGET" -p hiy-server 2>/dev/null || true
+RUN cargo build --release -p hiy-server 2>/dev/null || true
 RUN rm -f server/src/main.rs
 
 # Build actual source.
 COPY server/src ./server/src
-RUN TARGET=$(cat /rust_target) && \
-    touch server/src/main.rs && \
-    cargo build --release --target "$TARGET" -p hiy-server
-
-# Normalise binary location so the runtime stage doesn't need to know the target.
-RUN cp /build/target/"$(cat /rust_target)"/release/hiy-server /usr/local/bin/hiy-server
+RUN touch server/src/main.rs && \
+    cargo build --release -p hiy-server
 
 # ── Runtime stage ─────────────────────────────────────────────────────────────
 FROM docker.io/library/debian:bookworm-slim
@@ -78,7 +36,7 @@ RUN apt-get update && apt-get install -y \
     podman \
     && rm -rf /var/lib/apt/lists/*
 
-COPY --from=builder /usr/local/bin/hiy-server /usr/local/bin/hiy-server
+COPY --from=builder /build/target/release/hiy-server /usr/local/bin/hiy-server
 
 WORKDIR /app