feat: add 7-day free trial before paid stream billing

[ScriptedBro]

Summary

This PR adds a 7-day free trial window for streams. During the trial period, no charges accrue; after the trial ends, collection resumes at the configured paid rate.

What Changed

• Added a Tier struct with:
  • rate_per_second
  • trial_duration
• Updated Stream to store tier instead of a raw rate_per_second.
• Set default trial duration to 7 days on new subscriptions.
• Updated claim/collect logic to ignore elapsed time inside the trial period.
• Kept existing stream and group-channel flows compatible with the new tier model.
• Updated migrate_tier to read/write stream.tier.rate_per_second.
• Added and updated tests to validate:
  • no charges during trial
  • charges start after trial ends
  • existing flows (cancel/top-up/group/pause/tier migration) still behave correctly with trial-aware timing
• Updated README with the 7-day free trial behavior.

Why

This supports growth and onboarding by allowing creators to offer a free week before paid streaming begins.

Validation

• Ran cargo test
• Result: 11 passed, 0 failed

Checklist

• Added trial_duration to Tier
• Modified claim logic to ignore time delta during trial period
• Added/updated tests for free trial and regression coverage
• Updated README documentation

Issue

Closes #4
Closes: #4

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