feat: add initial CGI support with environment variable passing and user permission fields

Implement core CGI execution infrastructure including cgi_config_init, cgi_is_executable, cgi_detect_headers, and cgi_execute functions in new src/cgi/cgi.c module. Extend auth_user_t struct with cgi_enabled, cgi_timeout, cgi_max_output, webserve_enabled, directory_listing, default_index, and granular read/write/execute permission fields. Add HTTP_206_PARTIAL_CONTENT, HTTP_416_RANGE_NOT_SATISFIABLE, and HTTP_500_INTERNAL_ERROR status codes to enum. Introduce streaming configuration block with buffer sizes, sendfile threshold, and per-request memory limit in config struct. Add pathmapping.c to both server and CLI build sources. Extend CLI webdavctl with user directory, permission, and CGI management subcommands. Expose auth_get_db function and sqlite3 forward declaration in auth.h.

include/nwedav.h

@@ -117,6 +117,7 @@ typedef enum {
     HTTP_200_OK = 200,
     HTTP_201_CREATED = 201,
     HTTP_204_NO_CONTENT = 204,
+    HTTP_206_PARTIAL_CONTENT = 206,
     HTTP_207_MULTI_STATUS = 207,
     HTTP_301_MOVED_PERMANENTLY = 301,
     HTTP_302_FOUND = 302,
@@ -130,8 +131,10 @@ typedef enum {
     HTTP_412_PRECONDITION_FAILED = 412,
     HTTP_413_PAYLOAD_TOO_LARGE = 413,
     HTTP_415_UNSUPPORTED_MEDIA_TYPE = 415,
+    HTTP_416_RANGE_NOT_SATISFIABLE = 416,
     HTTP_423_LOCKED = 423,
     HTTP_424_FAILED_DEPENDENCY = 424,
+    HTTP_500_INTERNAL_ERROR = 500,
     HTTP_500_INTERNAL_SERVER_ERROR = 500,
     HTTP_501_NOT_IMPLEMENTED = 501,
     HTTP_502_BAD_GATEWAY = 502,