core (launcher): Separate add_subparsers for exensibility:

`add_subparsers` is now in its own method to it can be extended by a subclass. Notably
needed by web frontend to add its `local` command.

libervia/backend/core/launcher.py

@@ -21,6 +21,7 @@
 import sys
 import os
 import argparse
+from argparse import _SubParsersAction
 from pathlib import Path
 
 # We want to install asyncreactor before any import from twisted or Libervia to be sure
@@ -170,10 +171,25 @@ class Launcher:
             print(self.NOT_RUNNING_MSG)
             sys.exit(1)
 
-    def parse_args(self):
+    def parse_args(self) -> argparse.Namespace:
+        """Parse command-line arguments and return the resulting namespace.
+
+        Subclasses can override to add custom subparsers before calling
+        ``parser.parse_args()``.
+
+        @return: The configured argument parser.
+        """
         parser = argparse.ArgumentParser(description=f"Launch {self.APP_NAME} backend")
         parser.set_defaults(cmd=self.cmd_no_subparser)
-        subparsers = parser.add_subparsers()
+        subparsers: _SubParsersAction = parser.add_subparsers()
+        self.add_subparsers(subparsers)
+        return parser.parse_args()
+
+    def add_subparsers(self, subparsers: _SubParsersAction) -> None:
+        """Add subparsers (bg, fg, debug, stop, status).
+
+        @param subparsers: The subparsers object from the argument parser.
+        """
         extra_help = f"arguments to pass to {self.APP_NAME} service"
 
         bg_parser = subparsers.add_parser(
@@ -208,8 +224,6 @@ class Launcher:
         )
         status_parser.set_defaults(cmd=self.cmd_status)
 
-        return parser.parse_args()
-
     def get_pid_file(self) -> Path:
         return self.config.common.pid_dir / f"{self.APP_NAME_FILE}.pid"