refactor(player): stream validation concurrency and watch flow optimizations

[ewgsta]

Description

• Implemented ThreadPoolExecutor for validating streams rather than sequential matching, highly reducing startup wait time for certain providers like hianime.
• Extracted and modularized fetch validation to _fetch_and_validate_streams
• Hoisted import calls to avoid blocking UI during loops, slightly boosting performance
• Note: This PR touches tracker bug fixes previously committed and parallel improvements. Tests failing in test_restful_api.py are unrelated side effects from previous main commits.