Debounce movie and series search with SwiftUI tasks

[tillkruss]

Summary

• Replace the local Combine search publishers in movie and series search with SwiftUI task(id:) driven search requests
• Add a small SearchRequest value to keep repeated query/debounce state readable
• Keep typed search input debounced while allowing submit and initial non-empty queries to run immediately
• Leave unrelated alert fallback and discovery-grid logic unchanged

Verification

• Ran git diff --check
• Did not run an Xcode build