fix: tolerate concurrent user creation

[vycdev2]

Summary

• Handle a unique-key race when concurrent Discord message handlers create the same user.
• Clear the failed EF change tracker and reload the persisted user instead of failing activity processing.
• Add a SQLite regression test that simulates the competing insert.

Verification

• dotnet restore Morpheus.sln — passed (existing NU1903 SQLite vulnerability warning).
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter FullyQualifiedName~UsersServiceTests — passed: 1/1.
• dotnet build Morpheus.sln --no-restore — passed: 0 errors (existing NU1903 warning).
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-build --no-restore — 298 passed, 2 failed: existing tr-TR culture cases unavailable in this runner's globalization-invariant mode.
• git diff --check upstream/develop...HEAD — passed.

Risk

• Low — the change only recovers from a concurrent unique-key conflict and preserves the existing creation path otherwise.

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