feature: upgrade to .NET 8, add request validation and RFC 7807 problem details

[devin-ai-integration]

Summary

.NET 8 upgrade: all projects → net8.0; EF Core / Npgsql / ASP.NET packages → 8.0.x; test stack → xunit 2.9.2, FakeItEasy 8.3, coverlet 6.0.2; Dockerfile → aspnet:8.0/sdk:8.0; CI workflow → setup-dotnet@v4 with 8.0.x (was pinned to 6.0.x) and collects XPlat coverage.

Request validation: EducationDto gets DataAnnotations mirroring the EF config (Degree ≤50, FieldOfStudy/School ≤250 required, Description ≤1000). The required C# modifier was dropped (= string.Empty) so a missing field yields a 400 ValidationProblemDetails keyed by property instead of a System.Text.Json deserialization error.

RFC 7807 everywhere:

• AddProblemDetails() with traceId extension; new GlobalExceptionHandler : IExceptionHandler (.NET 8) returns a 500 application/problem+json without leaking the exception message; UseExceptionHandler() + UseStatusCodePages().
• EducationsController drops all try/catch and uses Problem(...)/ValidationProblem(ModelState).

Controller contract changes (behavioral, please review):

Endpoint	Before	After
GET /{id} missing	204	404 problem
POST ok	200	201 + Location
PUT/DELETE ok	200	204
PUT/DELETE missing	400 (empty)	404 problem
PUT id mismatch	400 (empty)	400 problem
any exception	500 (empty)	500 problem+json

Health check registration is skipped when no connection string is configured (needed for WebApplicationFactory tests).

Tests: controller unit tests rewritten (real HttpContext so Problem() resolves ProblemDetailsFactory) plus EducationsApiIntegrationTests over HTTP via WebApplicationFactory<Program> asserting application/problem+json, error keys, traceId, Location header. EducationsController line coverage: 100% (WebApi assembly 96%). 22 tests pass.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/5974d55e53804be082172182ac4a9283
Open in Devin Desktop: https://app.devin.ai/desktop/session/5974d55e53804be082172182ac4a9283?variant=devin
Requested by: @rushcromer

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

[devin-ai-integration]

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