refactor: reorganize into subdirs (config/ util/ http/ notifs/ queue/ integrations/ static/)

util/buf.h

@@ -0,0 +1,10 @@
+#pragma once
+#include <stddef.h>
+
+typedef struct { char *data; size_t len; size_t cap; } Buf;
+
+void buf_init(Buf *b);
+void buf_free(Buf *b);
+void buf_append(Buf *b, const char *s, size_t n);
+void buf_str(Buf *b, const char *s);
+void buf_fmt(Buf *b, const char *fmt, ...) __attribute__((format(printf,2,3)));