feat: implement post-init function for application startup tasks

- Added _post_init function to run startup tasks, including restoring group pin jobs and resolving bot username.
- Updated main function to utilize the new _post_init for improved application initialization process.

duty_teller/run.py

@@ -19,6 +19,12 @@ from duty_teller.utils.http_client import safe_urlopen
 _HTTP_STARTUP_WAIT_SEC = 3
 
 
+async def _post_init(application) -> None:
+    """Run startup tasks: restore group pin jobs, then resolve bot username."""
+    await group_duty_pin.restore_group_pin_jobs(application)
+    await _resolve_bot_username(application)
+
+
 async def _resolve_bot_username(application) -> None:
     """If BOT_USERNAME is not set from env, resolve it via get_me()."""
     if not config.BOT_USERNAME:
@@ -101,8 +107,7 @@ def main() -> None:
     app = (
         ApplicationBuilder()
         .token(config.BOT_TOKEN)
-        .post_init(group_duty_pin.restore_group_pin_jobs)
+        .post_init(_post_init)
-        .post_init(_resolve_bot_username)
         .build()
     )
     register_handlers(app)