chore: add governing repository license

[peter-matkovski]

Summary

• add a governing repository-level Stream Source Code License
• clarify the existing package-specific license boundaries in the README
• preserve stream_thumbnail under its existing MIT License

Contributor-rights check

All human contributors are Stream staff; automated contributions are attributable to Stream-controlled tooling.

Validation

• the new root license matches packages/stream_core/LICENSE byte-for-byte
• the existing licenses for stream_core, stream_core_flutter, and stream_thumbnail are unchanged
• non-license changes pass git diff --check

Tracking: APPSEC-316

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.70%. Comparing base (ab4b1fb) to head (7016158).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   54.70%   54.70%           
=======================================
  Files         185      185           
  Lines        7575     7575           
=======================================
  Hits         4144     4144           
  Misses       3431     3431           

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