refactor(examples): S13 PR 4 — .NET on EF Core, HealthChecks and BackgroundService

[patterncatalyst]

S13 PR 4 of 5 (plan: _plans/s13-idiomatic-plan.md, step 5).

• The .NET examples move from hand-built Npgsql commands to EF Core, with db/init still owning the schema (no migrations, no EnsureCreated). 04's outbox is written in a single SaveChanges, and 17's saga locks its row with FromSql ... FOR UPDATE.
• Hand-written MapGet health endpoints are replaced by ASP.NET Core HealthChecks on /healthz and /readyz.
• Console consumers and the 25 flusher become BackgroundServices. 21 drains through IHostedLifecycleService instead of PosixSignalRegistration.
• Configuration moves to IConfiguration/Options (D5), new HttpClient to typed HttpClients, and 18's errors to AddProblemDetails / TypedResults.Problem.
• 25 uses HybridCache for read-through only; the other strategies keep explicit Redis clients (D4).
• 19's CLI runs on a plain host without Kestrel. 11 and 16 enable nullable reference types.
• The .NET site tabs, READMEs and deck (DotNet.fix6.json) follow the new code.

The rebase onto main hit one conflict in _docs/25-appendix-l-caching.md (read-through prose). It now keeps Quarkus's Cache.get from PR 2 and names .NET HybridCache's GetOrCreateAsync.

Verification: scripts/verify-matrix.sh dotnet passed 19/19. validate_site, voice-scan and deck_text --check report clean.