Modify get_participants() in main.py with pagination and async generator for large groups

[chigwell]

User Story
As a user managing large Telegram groups,
I want paginated access to group participants
so that I can reliably retrieve complete membership lists without data loss.

Background
The current get_participants() implementation in main.py uses Telethon's default limit, risking incomplete data retrieval in groups with >200 members. This violates data completeness requirements and prevents analysis of large communities. The function currently:

• Calls client.get_participants() without pagination parameters
• Returns results as immediate list
• Lacks control over batch sizes

Key risks:

1. Telegram API limits (400 users per request default)
2. Memory issues with large participant lists
3. Timeouts in massive groups

Acceptance Criteria

• Modify main.py's get_participants() tool implementation
• Add page (1-indexed) and page_size (default 200) parameters
• Implement pagination using Telethon's limit and offset parameters
• Convert to async generator using client.iter_participants()
• Add safety limit (max 1000 participants/page)
• Preserve current text output format with pagination metadata

Validation:

1. Test with group containing 1500 participants - verify 8 pages (7x200 + 1x100)
2. Confirm no duplicate IDs across consecutive pages
3. Verify error handling for invalid page numbers
4. Check memory usage stays under 50MB for 1000-user batches

Implementation Note:
Leverage Telethon's async for participant in client.iter_participants() with limit and offset for efficient pagination while maintaining session state.