Fix: Resolve webhook send timeout context error

ISSUE
=====
When using the manual webhook message feature via API, the following error occurred:
- 'Timeout context manager should be used inside a task'
- 'NoneType' object is not iterable (when sending without files)

The error happened because Discord.py's webhook operations were being awaited
directly in the FastAPI endpoint context, rather than within a task running in
the bot's event loop.

SOLUTION
========
Refactored /manual/send-webhook endpoint to properly handle async operations:

1. Moved webhook creation inside task function
   - get_or_create_webhooks_for_channel() now runs in send_webhook_message()
   - All Discord operations (webhook selection, sending) happen inside the task
   - Follows same pattern as working /manual/send endpoint

2. Fixed file parameter handling
   - Changed from 'files=discord_files if discord_files else None'
   - To conditional: only pass files parameter when list is non-empty
   - Discord.py's webhook.send() cannot iterate over None, requires list or omit

3. Maintained proper file reading
   - File content still read in endpoint context (before form closes)
   - File data passed to task as pre-read byte arrays
   - Prevents form closure issues

TECHNICAL DETAILS
=================
- Discord.py HTTP operations use timeout context managers
- Context managers must run inside bot's event loop (via create_task)
- FastAPI endpoint context is separate from bot's event loop
- Solution: Wrap all Discord API calls in async task function
- Pattern: Read files → Create task → Task handles Discord operations

TESTING
=======
- Manual webhook sending now works without timeout errors
- Both personas (Miku/Evil) send correctly
- File attachments work properly
- Messages without files send correctly

bot/api.py

@@ -866,15 +866,6 @@ async def manual_send_webhook(
         if persona not in ["miku", "evil"]:
             return {"status": "error", "message": "Invalid persona. Must be 'miku' or 'evil'"}
         
-        # Get or create webhooks for this channel
-        webhooks = await get_or_create_webhooks_for_channel(channel)
-        if not webhooks:
-            return {"status": "error", "message": "Failed to create webhooks for this channel"}
-        
-        # Select the appropriate webhook
-        webhook = webhooks["evil_miku"] if persona == "evil" else webhooks["miku"]
-        display_name = get_evil_miku_display_name() if persona == "evil" else get_miku_display_name()
-        
         # Read file content immediately before the request closes
         file_data = []
         for file in files:
@@ -891,24 +882,43 @@ async def manual_send_webhook(
         # Use create_task to avoid timeout context manager error
         async def send_webhook_message():
             try:
+                # Get or create webhooks for this channel (inside the task)
+                webhooks = await get_or_create_webhooks_for_channel(channel)
+                if not webhooks:
+                    print(f"❌ Failed to create webhooks for channel #{channel.name}")
+                    return
+                
+                # Select the appropriate webhook
+                webhook = webhooks["evil_miku"] if persona == "evil" else webhooks["miku"]
+                display_name = get_evil_miku_display_name() if persona == "evil" else get_miku_display_name()
+                
                 # Prepare files for webhook
                 discord_files = []
                 for file_info in file_data:
                     discord_files.append(discord.File(io.BytesIO(file_info['content']), filename=file_info['filename']))
                 
                 # Send via webhook with display name
-                await webhook.send(
-                    content=message,
-                    username=display_name,
-                    files=discord_files if discord_files else None,
-                    wait=True
-                )
+                if discord_files:
+                    await webhook.send(
+                        content=message,
+                        username=display_name,
+                        files=discord_files,
+                        wait=True
+                    )
+                else:
+                    await webhook.send(
+                        content=message,
+                        username=display_name,
+                        wait=True
+                    )
                 
                 persona_name = "Evil Miku" if persona == "evil" else "Hatsune Miku"
                 print(f"✅ Manual webhook message sent as {persona_name} to #{channel.name}")
                 
             except Exception as e:
                 print(f"❌ Failed to send webhook message: {e}")
+                import traceback
+                traceback.print_exc()
         
         globals.client.loop.create_task(send_webhook_message())
         return {"status": "ok", "message": f"Webhook message queued for sending as {persona}"}