fix(api): align health endpoint with API conventions (#108)

Fix the WIP health-check API slice to match established patterns:

- HealthController: add Name="GetHealthChecks" route name and move
  [EndpointGroupName("general")] to method level, matching
  FillerPresetController/FFmpegProfileController exactly (the
  precedent for parameterless 200-only GET actions).
- HealthCheckResponseModel: enable #nullable for the file and mark
  Link as string? since the mapper can emit null when
  HealthCheckResult.Link is None.
- Mapper: fix a real bug - LanguageExt's Option.Match throws
  ResultIsNullException.ResultIsNull if either branch returns null
  (by design, to catch accidental nulls). The WIP's
  `Link.Match(l => l.Link, () => null)` crashed on every health
  check without a Link. Switch to MatchUnsafe, the LanguageExt-
  sanctioned way to intentionally produce a nullable result from
  Option<T>.
- HealthControllerTests: add the idiomatic-route-assertion test
  (route template + Name) and empty-list case, matching
  FillerPresetControllerTests.

Verified: GetAllHealthCheckResultsForApiHandler already matches the
existing GetAllHealthCheckResultsHandler's cancellation handling
(both swallow TaskCanceledException/OperationCanceledException), so
no change was needed there. Confirmed no OpenApi contract test
enumerates all endpoints for error-response metadata (it's an
explicit TestCase allowlist), so the new GET needed no new entries.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV/Controllers/Api/HealthController.cs

@@ -6,12 +6,12 @@ using Microsoft.AspNetCore.Mvc;
 namespace ErsatzTV.Controllers.Api;
 
 [ApiController]
-[EndpointGroupName("general")]
 public class HealthController(IMediator mediator) : ControllerBase
 {
-    [HttpGet("/api/health")]
+    [HttpGet("/api/health", Name = "GetHealthChecks")]
     [Tags("Health")]
     [EndpointSummary("Get health check results")]
+    [EndpointGroupName("general")]
     [ProducesResponseType(typeof(List<HealthCheckResponseModel>), StatusCodes.Status200OK)]
     public async Task<List<HealthCheckResponseModel>> GetAll(CancellationToken cancellationToken) =>
         await mediator.Send(new GetAllHealthCheckResultsForApi(), cancellationToken);