v0.3.3 - Fix function naming consistency: rename find_existing_nrdb_files to find_existing_db_files

- Update function declaration in config.h
- Update function definition in config.c
- Update function calls in config.c and main.c
- Maintain consistency with .db file extension naming convention

This resolves the inconsistency between database file extension (.db) and function names (nrdb)

src/main.c

@@ -3251,7 +3251,7 @@ int main(int argc, char* argv[]) {
         log_info("Existing relay detected");
         
         // Find existing database file
-        char** existing_files = find_existing_nrdb_files();
+        char** existing_files = find_existing_db_files();
         if (!existing_files || !existing_files[0]) {
             log_error("No existing relay database found");
             nostr_cleanup();