Add MUSL static binary build system using Alpine Docker

- Create Dockerfile.alpine-musl for truly portable static binaries
- Update build_static.sh to use Docker with sudo fallback
- Fix source code portability issues for MUSL:
  * Add missing headers in config.c, dm_admin.c
  * Remove glibc-specific headers in nip009.c, subscriptions.c
- Update nostr_core_lib submodule with fortification fix
- Add comprehensive documentation in docs/musl_static_build.md

Binary characteristics:
- Size: 7.6MB (vs 12MB+ for glibc static)
- Dependencies: Zero (truly portable)
- Compatibility: Any Linux distribution
- Build time: ~2 minutes with Docker caching

Resolves fortification symbol issues (__snprintf_chk, __fprintf_chk)
that prevented MUSL static linking.

Dockerfile.alpine-musl

@@ -0,0 +1,109 @@
+# Alpine-based MUSL static binary builder for C-Relay
+# Produces truly portable binaries with zero runtime dependencies
+
+FROM alpine:3.19 AS builder
+
+# Install build dependencies
+RUN apk add --no-cache \
+    build-base \
+    musl-dev \
+    git \
+    cmake \
+    pkgconfig \
+    autoconf \
+    automake \
+    libtool \
+    openssl-dev \
+    openssl-libs-static \
+    zlib-dev \
+    zlib-static \
+    curl-dev \
+    curl-static \
+    sqlite-dev \
+    sqlite-static \
+    linux-headers \
+    wget \
+    bash
+
+# Set working directory
+WORKDIR /build
+
+# Build libsecp256k1 static
+RUN cd /tmp && \
+    git clone https://github.com/bitcoin-core/secp256k1.git && \
+    cd secp256k1 && \
+    ./autogen.sh && \
+    ./configure --enable-static --disable-shared --prefix=/usr \
+        CFLAGS="-fPIC" && \
+    make -j$(nproc) && \
+    make install && \
+    rm -rf /tmp/secp256k1
+
+# Build libwebsockets static with minimal features
+RUN cd /tmp && \
+    git clone --depth 1 --branch v4.3.3 https://github.com/warmcat/libwebsockets.git && \
+    cd libwebsockets && \
+    mkdir build && cd build && \
+    cmake .. \
+        -DLWS_WITH_STATIC=ON \
+        -DLWS_WITH_SHARED=OFF \
+        -DLWS_WITH_SSL=ON \
+        -DLWS_WITHOUT_TESTAPPS=ON \
+        -DLWS_WITHOUT_TEST_SERVER=ON \
+        -DLWS_WITHOUT_TEST_CLIENT=ON \
+        -DLWS_WITHOUT_TEST_PING=ON \
+        -DLWS_WITH_HTTP2=OFF \
+        -DLWS_WITH_LIBUV=OFF \
+        -DLWS_WITH_LIBEVENT=OFF \
+        -DLWS_IPV6=ON \
+        -DCMAKE_BUILD_TYPE=Release \
+        -DCMAKE_INSTALL_PREFIX=/usr \
+        -DCMAKE_C_FLAGS="-fPIC" && \
+    make -j$(nproc) && \
+    make install && \
+    rm -rf /tmp/libwebsockets
+
+# Copy c-relay source
+COPY . /build/
+
+# Clean up any stale submodule references (nips directory is not a submodule)
+RUN git rm --cached nips 2>/dev/null || true
+
+# Initialize submodules and build nostr_core_lib with required NIPs
+# Disable fortification in build.sh to prevent __*_chk symbol issues
+# NIPs: 001(Basic), 006(Keys), 013(PoW), 017(DMs), 019(Bech32), 044(Encryption), 059(Gift Wrap - required by NIP-17)
+RUN git submodule update --init --recursive && \
+    cd nostr_core_lib && \
+    chmod +x build.sh && \
+    sed -i 's/CFLAGS="-Wall -Wextra -std=c99 -fPIC -O2"/CFLAGS="-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -Wall -Wextra -std=c99 -fPIC -O2"/' build.sh && \
+    rm -f *.o *.a 2>/dev/null || true && \
+    ./build.sh --nips=1,6,13,17,19,44,59
+
+# Build c-relay with full static linking
+# Disable fortification to avoid __*_chk symbols that don't exist in MUSL
+RUN gcc -static -O2 -Wall -Wextra -std=c99 \
+    -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 \
+    -I. -Inostr_core_lib -Inostr_core_lib/nostr_core \
+    -Inostr_core_lib/cjson -Inostr_core_lib/nostr_websocket \
+    src/main.c src/config.c src/dm_admin.c src/request_validator.c \
+    src/nip009.c src/nip011.c src/nip013.c src/nip040.c src/nip042.c \
+    src/websockets.c src/subscriptions.c src/api.c src/embedded_web_content.c \
+    -o /build/c_relay_static_musl \
+    nostr_core_lib/libnostr_core_x64.a \
+    -lwebsockets -lssl -lcrypto -lsqlite3 -lsecp256k1 \
+    -lcurl -lz -lpthread -lm -ldl
+
+# Strip binary to reduce size
+RUN strip /build/c_relay_static_musl
+
+# Verify it's truly static
+RUN echo "=== Binary Information ===" && \
+    file /build/c_relay_static_musl && \
+    ls -lh /build/c_relay_static_musl && \
+    echo "=== Checking for dynamic dependencies ===" && \
+    (ldd /build/c_relay_static_musl 2>&1 || echo "Binary is static") && \
+    echo "=== Build complete ==="
+
+# Output stage - just the binary
+FROM scratch AS output
+COPY --from=builder /build/c_relay_static_musl /c_relay_static_musl