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.

src/dm_admin.c

@@ -8,6 +8,7 @@
 #include <string.h>
 #include <strings.h>
 #include <unistd.h>
+#include <sys/stat.h>
 #include <cjson/cJSON.h>
 #include <libwebsockets.h>