fix: tolerate concurrent channel creation

[vycdev2]

Summary

• Handle the unique-key race when multiple handlers first observe the same Discord channel.
• Clear the failed insert, reload the winning row, and preserve the existing channel-name update behavior.
• Add a SQLite regression test that simulates a competing channel creation; no schema changes.

Verification

• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter 'FullyQualifiedName~ChannelServiceConcurrencyTests' — passed: 1/1 after the fix; the same test failed before the fix with the simulated DbUpdateException.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter 'FullyQualifiedName~ChannelService' — passed: 2/2.
• dotnet build --no-restore — passed: 0 errors; one existing NU1903 SQLite vulnerability warning.
• dotnet format Morpheus.sln --no-restore --verify-no-changes --include Services/ChannelService.cs Morpheus.Tests/ChannelServiceConcurrencyTests.cs — passed.
• dotnet test --no-restore — 298 passed, 2 failed: existing MiscModuleTests.NormalizeTimeUntilEventName_NormalizesCase cases cannot load tr-TR under this runner's globalization-invariant mode.
• git diff --check — passed.

Risk

Low: the change only handles a database unique-key conflict during channel creation and reuses the existing unique DiscordId index; unrelated database errors are rethrown when no competing row is found.

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