Upgrade solution from .NET 7 to .NET 9

[devin-ai-integration]

Summary

Retargets all six projects to net9.0 and moves the dependency stack to the current stable releases (EF Core / OpenAPI 9.0.19, Npgsql 9.0.x, HealthChecks.NpgSql 9.0.0, Swashbuckle 10.2.3, Mapster 10.0.12, test stack incl. xunit 2.9.3 — deliberately staying on xunit v2 to avoid a v3 test-code migration). Docker images and the CI workflow move to 9.0 (CI was still pinned to 6.0.x), and global.json pins the SDK to 9.0.317 with rollForward: latestFeature.

Only one code change was needed: GetConnectionString is nullable, so under the 9.x overloads the connection string is now resolved once and validated instead of being passed through twice:

-options.UseNpgsql(builder.Configuration.GetConnectionString("DefaultConnection"), ...)
-...AddNpgSql(builder.Configuration.GetConnectionString("DefaultConnection"), name: "SampleDB")
+var connectionString = builder.Configuration.GetConnectionString("DefaultConnection")
+    ?? throw new InvalidOperationException("DefaultConnection is not configured.");

Swagger stays Swashbuckle-based (not switched to the new Microsoft.AspNetCore.OpenApi document generation).

dotnet restore / build / test on .NET 9: build succeeded (2 pre-existing nullable warnings in EducationsControllerTests), 20/20 tests pass.

Link to Devin session: https://app.devin.ai/sessions/fa161b241b3d430fa67770922b66e680
Open in Devin Desktop: https://app.devin.ai/desktop/session/fa161b241b3d430fa67770922b66e680?variant=devin
Requested by: @detectiveharree

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Note

Devin errored when opening this Pull Request as detectiveharree.
As a fallback, Devin opened this PR as itself.

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