fix: implement actual maintenance mode flag in /admin-maintenance (#28)

- Add `maintenance_mode: bool = False` flag to `SBABot.__init__`
- Register a global `@tree.interaction_check` that blocks non-admin users
  from all commands when maintenance mode is active
- Update `admin_maintenance` command to set `self.bot.maintenance_mode`
  and log the state change, replacing the no-op placeholder comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot.py

@@ -81,11 +81,28 @@ class SBABot(commands.Bot):
         )
 
         self.logger = logging.getLogger("discord_bot_v2")
+        self.maintenance_mode: bool = False
 
     async def setup_hook(self):
         """Called when the bot is starting up."""
         self.logger.info("Setting up bot...")
 
+        @self.tree.interaction_check
+        async def maintenance_check(interaction: discord.Interaction) -> bool:
+            """Block non-admin users when maintenance mode is enabled."""
+            if not self.maintenance_mode:
+                return True
+            if (
+                isinstance(interaction.user, discord.Member)
+                and interaction.user.guild_permissions.administrator
+            ):
+                return True
+            await interaction.response.send_message(
+                "🔧 The bot is currently in maintenance mode. Please try again later.",
+                ephemeral=True,
+            )
+            return False
+
         # Load command packages
         await self._load_command_packages()