feat: Add V2 Hype Train EventSub events

[mphacker]

Summary

Adds support for V2 Hype Train EventSub events (channel.hype_train.begin, .progress, .end), matching the current Twitch EventSub API which has moved to version 2.

Changes

New V2 fields added:

• type (string): regular, treasure, golden_kappa - the Hype Train variant
• is_shared_train (boolean): whether this is a shared Hype Train
• shared_train_participants (array|null): list of broadcasters in a shared train
• all_time_high_level / all_time_high_total (begin event only)

V2 fields removed vs V1:

• last_contribution removed (only top_contributions remains in V2)

Files changed:

• internal/models/hype_train.go - Added V2 model structs
• internal/events/types/hype_train_v2/hype_train_event.go - New V2 event handler
• internal/events/types/hype_train_v2/hype_train_event_test.go - Tests for all triggers
• internal/events/types/types.go - Registered V2, deprecated V1
• docs/event.md - Updated documentation

Design decisions

• Backward compatible: V1 preserved; V2 is a separate package following the channel_update_v1/v2 pattern
• V1 marked as removed: Added to RemovedEvents() since Twitch no longer lists V1
• Uses only public API: All fields from official Twitch EventSub docs

Testing

• All existing tests pass (go test ./...)
• New V2 tests cover begin, progress, end triggers for webhook and websocket
• Cross-compiled and verified on Windows x64