Migrate solution from .NET 7 to .NET 9

[devin-ai-integration]

Summary

Upgrades the entire clean-architecture solution from net7.0 to net9.0 and bumps all NuGet packages to their .NET 9-compatible releases.

• Target framework: net7.0 → net9.0 in all six projects (Domain, Application, Infrastructure, WebApi, Application.Tests, WebApi.Tests).
• WebApi packages: AspNetCore.HealthChecks.NpgSql 7.0.0 → 9.0.0, Microsoft.AspNetCore.OpenApi 7.0.9 → 9.0.7, Microsoft.EntityFrameworkCore.Design 7.0.10 → 9.0.7, Swashbuckle.AspNetCore 6.5.0 → 7.2.0.
• Infrastructure packages: EF Core (Core/Design/SqlServer/Tools) 7.0.10 → 9.0.7; Npgsql 7.0.4 → 9.0.3, Npgsql.EntityFrameworkCore.PostgreSQL(.NodaTime) 7.0.4 → 9.0.4.
• Test packages: Microsoft.NET.Test.Sdk → 17.12.0, xunit → 2.9.3, xunit.runner.visualstudio → 3.0.2, coverlet.collector → 6.0.4, FakeItEasy → 8.3.0.
• Dockerfile: aspnet:7.0/sdk:7.0 → 9.0.
• CI (.github/workflows/dotnet.yml): dotnet-version 6.0.x → 9.0.x (previously inconsistent with the project TFM); actions/checkout@v3 → v4, actions/setup-dotnet@v3 → v4.
• Docs: README.md and GRAPHQL_SPEC.md references updated from .NET 7 to .NET 9. Repository not renamed (clone URL left intact).

Breaking-change review

• OpenAPI: Program.cs uses Swashbuckle (AddSwaggerGen/UseSwagger/UseSwaggerUI) with AddEndpointsApiExplorer, not the new .NET 9 AddOpenApi/MapOpenApi model — no code change required.
• Npgsql/EF Core 9 + NodaTime: UseNodaTime() still applies via the NodaTime plugin; domain entities use plain Guid/string (no NodaTime CLR types), so type-mapping changes have no code impact.

Verification

dotnet restore, dotnet build, and dotnet test all succeed on .NET 9 SDK 9.0.315 — 20/20 tests pass (6 Application.Tests, 14 WebApi.Tests). Remaining build warnings (nullable CS8604/CS8625) are pre-existing and unrelated to the migration.

Link to Devin session: https://app.devin.ai/sessions/6db7132c8bd14fdb8ecfb77d6617ac09
Requested by: @detectiveharree

---

Devin Review

Status	Commit
⚪ Not started	—

Run Devin Review

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring