fix(api): surface programs not filler in channel state, tighten queries

Adversarial review fixes for the #97 endpoint:

- nowPlaying now resolves like the XMLTV guide: when the wall-clock item
  is pre/mid-roll filler, surface the guide group's program item (title +
  full program bounds) instead of the filler; a guide entry with no
  program item (e.g. fallback loop) reports null. Previously an ad break
  would retitle the channel and reset progress every 30 seconds.
- covering-item lookup now runs one cheap projected query per distinct
  playout offset instead of hydrating the full metadata include tree for
  a window widened by the largest offset across ALL channels (up to 24h);
  metadata is fetched in a second query scoped to the covering guide
  groups, time-bounded to stay safe against GuideGroup recycling (mod
  10000). Regression caught while testing: covering items are keyed by
  (source channel, lookup time) so a source channel no longer inherits
  its offset mirror's lookup result.
- GetChannelStatesForApi now carries Now explicitly (controller passes
  DateTime.UtcNow), making boundary semantics testable: new tests pin
  now == Start (playing) and now == Finish (not playing), plus mid-roll,
  filler-only, and mirror-source-row cases.
- ChannelNowPlayingResponseModel title is non-nullable in the schema
  (#nullable enable) — GetDisplayTitle always returns a string.
- GetState uses the typed-return controller convention; Playouts.Mapper
  reverted to internal (handler lives in the same assembly).
- Spec doc notes the OnDemand drift caveat and filler semantics.

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

ErsatzTV/Controllers/Api/ChannelController.cs

@@ -25,13 +25,8 @@ public class ChannelController(ChannelWriter<IBackgroundServiceRequest> workerCh
     [Tags("Channels")]
     [EndpointSummary("Get channel runtime state")]
     [EndpointGroupName("general")]
-    [ProducesResponseType(typeof(List<ChannelStateResponseModel>), StatusCodes.Status200OK)]
-    public async Task<IActionResult> GetState(CancellationToken cancellationToken)
-    {
-        List<ChannelStateResponseModel> result =
-            await mediator.Send(new GetChannelStatesForApi(), cancellationToken);
-        return new OkObjectResult(result);
-    }
+    public async Task<List<ChannelStateResponseModel>> GetState(CancellationToken cancellationToken) =>
+        await mediator.Send(new GetChannelStatesForApi(DateTime.UtcNow), cancellationToken);
 
     [HttpGet("/api/channels/{id:int}", Name = "GetChannelById")]
     [Tags("Channels")]