Upgrade assets-svc from .NET 8 to .NET 10 (LTS)

[GeekTrainer]

Part A — app baseline .NET 8 → .NET 10

Moves the modern assets-svc service to the current .NET 10 LTS. There is no pedagogical reason for .NET 8 — only the Java audit-svc is intentionally dated (the Module 6 modernization target). Java services are untouched.

Changes

• Target framework net8.0 → net10.0 in AssetsService.csproj and Tests/AssetsService.Tests.csproj.
• NuGet (reconciles/supersedes Dependabot Bump the assets-svc-dependencies group with 3 updates #30):
  • Dapper 2.1.35 → 2.1.79
  • Microsoft.Data.Sqlite 8.0.4 → 10.0.11
  • Swashbuckle.AspNetCore 6.6.2 → 10.2.3
  • Test deps → net10-compatible stable: Microsoft.AspNetCore.Mvc.Testing → 10.0.11, Microsoft.NET.Test.Sdk → 18.9.0, xunit → 2.9.3, xunit.runner.visualstudio → 3.1.5
• Devcontainer dotnet feature 8.0 → 10.0. devcontainer-lock.json regenerated via devcontainer upgrade — no change (the lock pins the feature installer, which is SDK-channel-agnostic and already resolves .NET 10).
• Dockerfile dotnet/sdk:8.0 + dotnet/aspnet:8.0 → :10.0 (functional: a net10 assembly won't run on the aspnet:8.0 runtime). Authored here, not by merging Dependabot Bump the docker-base-images group across 7 directories with 6 updates #29.
• Docs: README.md, CONTRIBUTING.md, services/assets-svc/README.md, PR template, and both issue templates .NET 8 → .NET 10.

Verification (net10; host dotnet 10.0.203)

• dotnet build — ✅ green (both projects target net10.0).
• dotnet test — ✅ green (2/2). Note: main only carries 2 test files; the "28 tests" figure is the post-delta learner-branch state and is validated in Part B.
• App start — ✅ dotnet run on :5001 serves /swagger/index.html (200), /swagger/v1/swagger.json (200), /assets (200); SQLite schema/seed init OK.
• Docker build not run on host (no daemon); Dockerfile images bumped for CI/devcontainer.

Scope / constraints

• No Java services, no course-build/ changes, no course workflow files.
• Does not merge Dependabot Bump the docker-base-images group across 7 directories with 6 updates #29 (Docker/temurin) or Bump the assets-svc-dependencies group with 3 updates #30 (this PR supersedes Bump the assets-svc-dependencies group with 3 updates #30 for assets-svc).

Note

Part B (learner-branch re-baseline) is planned separately and not in this PR. Because learner branches derive from the pinned tooling-free acc-base (not main), merging this does not move them. Part B re-baselines onto a new tooling-free base and re-derives the deltas (delta_01 + delta_05 hard-conflict on the net8 context). See the session plan; nothing there executes until this merges and Part B is approved.