feat: add channel pause controls for creator hiatus

[ScriptedBro]

Summary

This PR adds creator-controlled channel pause support so creators can temporarily stop all incoming stream charges while on hiatus, preventing fans from being billed during inactive periods.

What Changed

• Added creator channel pause state storage (ChannelPaused).
• Added new contract methods:
  • pause_channel
  • unpause_channel
  • is_channel_paused
• Updated collection logic to stop billing while a creator channel is paused.
• Ensured pause applies to the creator’s entire channel (all active subscribers).
• Updated pause/unpause flow so billing resumes from unpause time without charging paused intervals.
• Preserved existing stream functionality (subscribe, collect, cancel, top_up, migrate_tier, group flows).
• Added tests for:
  • pause blocks charges and unpause resumes correctly
  • pause applies across multiple subscribers
  • existing core stream behaviors remain valid
• Updated README key logic with pause/unpause behavior.

Why

Creators need a safe hiatus control so users are not charged when no content/service is being provided.

Validation

• Ran cargo test
• Result: 10 passed, 0 failed

Checklist

• Added creator-level pause/unpause channel controls
• Ensured paused periods are not charged
• Applied pause behavior across all incoming streams for a creator
• Added and updated tests for pause behavior and regressions
• Updated README documentation

Issue

Closes #19
Closes: #19

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