fix(api): rework from-lineup to generated playlist design + review fixes (#63)

Adversarial review found the previous multi-flood design non-viable:
PlayoutModeSchedulerFlood never yields to a following Dynamic-start
schedule item, so only the first item ever played, and grouping
media items into a Collection silently dropped the requested order.

Redesign:
- Single-item lineup: one ProgramScheduleItemFlood referencing the
  target directly (media item / collection / smart / multi / rerun /
  playlist); no generated collection or playlist. Response PlaylistId
  is null.
- Multi-item lineup (>= 2): one generated IsSystem Playlist in a
  get-or-created IsSystem PlaylistGroup ("Channel Lineups"), one
  PlaylistItem per entry in lineup order with PlayAll=true, referenced
  by a single Flood schedule item. Rerun collections and playlists are
  rejected (422) in multi lineups (PlaylistItem/CollectionKey lack the
  fields to enumerate them).

Review fixes:
- Normalize + strict-validate MediaType<->CollectionType pairs once up
  front (422 on mismatch / wrong id / not exactly one id).
- Reject MultiCollection with non-Shuffle order (mirrors
  PlayoutModeMustBeValid), Mirror playout source, all via 422.
- OnDemand parity: queue TimeShiftOnDemandPlayout post-commit.
- De-collide generated ProgramSchedule and Playlist names against their
  unique indexes instead of leaking a UNIQUE-constraint DbUpdateException.
- Generic 422 on save failure + ILogger; AnyAsync existence checks;
  Either/Validation unwrap via Match; XML doc on request DTO + endpoint.
- Response model: ChannelId, PlaylistId (nullable), ProgramScheduleId,
  PlayoutId (CollectionId removed). Regenerated OpenAPI v1.json + v1.d.ts.

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

ErsatzTV/Controllers/Api/ChannelController.cs

@@ -83,8 +83,15 @@ public class ChannelController(ChannelWriter<IBackgroundServiceRequest> workerCh
     [Tags("Channels")]
     [EndpointSummary("Create a channel from a library lineup")]
     [EndpointDescription(
-        "Atomically creates the channel, generated lineup collection, program schedule, schedule items, " +
-        "and classic playout. Template defaults are stamped at create time; advanced overrides win.")]
+        "Atomically creates the channel, program schedule, a classic playout, and (for multi-item lineups) a " +
+        "generated system playlist. A single-item lineup produces one flood schedule item that references the " +
+        "target directly (movie, show, season, artist, collection, smart/multi collection, rerun collection, or " +
+        "playlist) and no generated playlist. A lineup with two or more items produces one generated system " +
+        "playlist whose entries play in the given order (each entry played in full before the next) referenced by " +
+        "one flood schedule item; only movies, shows, seasons, artists, collections, smart collections and multi " +
+        "collections are allowed there (rerun collections and playlists are single-item only). playbackOrder sets " +
+        "how items within each lineup entry are ordered. Template defaults are stamped at create time; advanced " +
+        "overrides win.")]
     [EndpointGroupName("general")]
     [ProducesResponseType(typeof(CreateChannelFromLineupResponseModel), StatusCodes.Status201Created)]
     [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status404NotFound)]

ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs

@@ -8,6 +8,12 @@ using ErsatzTV.Core.Scheduling;
 
 namespace ErsatzTV.Controllers.Api.Requests;
 
+/// <summary>
+///     Composite request to create a channel, its generated schedule/playlist and a classic playout in one call.
+///     Template defaults are stamped at create time; any value set in <see cref="Advanced" /> overrides the template.
+///     A single-item lineup references its target directly; a multi-item lineup is played in order via a generated
+///     system playlist.
+/// </summary>
 public record CreateChannelFromLineupRequest(
     string Name,
     string Number,