Add maximum participant cap to campaign contract

[xqcxx]

Summary

Implements an optional maximum participant capacity feature for campaigns, allowing administrators to limit the number of participants who can register.

Changes

• Added CapacityReached error variant (Error code 102)
• Added storage for max capacity (MAX_CAP) and participant count (PARTICIPANT_COUNT)
• Implemented set_max_cap admin function to configure the capacity limit
• Updated register function to:
  • Check current count against max capacity before allowing registration
  • Automatically increment participant count on successful registration
  • Return clear error when capacity is reached
• Added get_participant_count query function to retrieve current participant count
• Added get_max_cap query function to retrieve the configured capacity limit

Implementation Details

The capacity system works as follows:

1. Default behavior: Max cap is 0 (unlimited participants) - maintains backward compatibility
2. Setting a cap: Admin calls set_max_cap with desired limit
3. Registration: The register function checks if max_cap > 0 and count >= max_cap before allowing new registrations
4. Counting: Participant count is tracked separately from individual participant records for efficient querying
5. Error handling: Returns CapacityReached error with clear message when limit is hit

Backward Compatibility

• Existing campaigns continue to work with unlimited participants (max_cap defaults to 0)
• No breaking changes to existing functions
• Optional feature that can be enabled per campaign

Closes #4

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