fix: preserve case-sensitive YouTube channel ids

[vycdev2]

Summary

• Preserve distinct case-sensitive YouTube channel IDs during bulk subscription parsing.
• Keep case-insensitive deduplication for Twitch handles and other subscription sources.
• Add regression coverage for IDs that differ only by letter case.

Verification

• dotnet restore — passed; existing NU1903 SQLite advisory warning.
• dotnet build --no-restore --nologo — passed; 0 errors.
• dotnet test --no-restore --filter FullyQualifiedName~SubscriptionInputParserTests --nologo — passed; 21/21.
• dotnet test --no-restore --nologo — 298/300 passed; 2 pre-existing culture tests fail because the environment is globalization-invariant and lacks ICU when disabled.
• dotnet format Morpheus.sln whitespace --no-restore --verify-no-changes — failed on the repository-wide pre-existing CRLF/whitespace baseline; git diff --check passed.

Risk

• Low: only bulk-source deduplication changes, with existing case-insensitive behavior retained for non-ID sources.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.