feat(api): add media-source write API shared DTOs (#202 slice S0)

Response DTOs (ErsatzTV.Core/Api/MediaSources) and request DTOs
(ErsatzTV/Controllers/Api/Requests) per the #202 design doc §B — the
shared shapes that backend slices S1 (local libraries), S2 (Plex), and
S3 (Jellyfin/Emby) will consume. No controllers or handler changes;
DTOs are unused so far.

Notable: RemoteConnectionResponseModel deliberately never carries the
raw API key (secure connection contract); SaveRemoteConnectionRequest's
To{Jellyfin,Emby}Command(existingApiKey) retains the existing key when
the incoming ApiKey is blank/omitted.

No conventions changed; nothing to update in docs/api-conventions.md.

ErsatzTV/Controllers/Api/Requests/CreateLocalLibraryRequest.cs

@@ -0,0 +1,9 @@
+using ErsatzTV.Application.Libraries;
+using ErsatzTV.Core.Domain;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+public record CreateLocalLibraryRequest(string Name, LibraryMediaKind MediaKind, List<string> Paths)
+{
+    public CreateLocalLibrary ToCommand() => new(Name, MediaKind, Paths ?? []);
+}

ErsatzTV/Controllers/Api/Requests/LocalPathCheckRequest.cs

@@ -0,0 +1,4 @@
+namespace ErsatzTV.Controllers.Api.Requests;
+
+// No command mapping — the controller checks filesystem existence directly (see design #202 §C2).
+public record LocalPathCheckRequest(string Path);

ErsatzTV/Controllers/Api/Requests/MoveLocalLibraryPathRequest.cs

@@ -0,0 +1,8 @@
+using ErsatzTV.Application.Libraries;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+public record MoveLocalLibraryPathRequest(int TargetLibraryId)
+{
+    public MoveLocalLibraryPath ToCommand(int pathId) => new(pathId, TargetLibraryId);
+}

ErsatzTV/Controllers/Api/Requests/PathReplacementItemRequest.cs

@@ -0,0 +1,5 @@
+namespace ErsatzTV.Controllers.Api.Requests;
+
+// Id 0/absent = new; existing Id = update; existing row whose Id is absent from the request = delete
+// (design #202 §C4b) — plus a per-row ownership guard and nonblank validation enforced by the handler.
+public record PathReplacementItemRequest(int Id, string RemotePath, string LocalPath);

ErsatzTV/Controllers/Api/Requests/RemoteLibraryPreferenceRequest.cs

@@ -0,0 +1,3 @@
+namespace ErsatzTV.Controllers.Api.Requests;
+
+public record RemoteLibraryPreferenceRequest(int Id, bool ShouldSyncItems);

ErsatzTV/Controllers/Api/Requests/ReplacePathReplacementsRequest.cs

@@ -0,0 +1,29 @@
+using ErsatzTV.Application.Emby;
+using ErsatzTV.Application.Jellyfin;
+using ErsatzTV.Application.Plex;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+public record ReplacePathReplacementsRequest(List<PathReplacementItemRequest> Items)
+{
+    public UpdatePlexPathReplacements ToPlexCommand(int sourceId) =>
+        new(
+            sourceId,
+            (Items ?? [])
+                .Select(item => new PlexPathReplacementItem(item.Id, item.RemotePath, item.LocalPath))
+                .ToList());
+
+    public UpdateJellyfinPathReplacements ToJellyfinCommand(int sourceId) =>
+        new(
+            sourceId,
+            (Items ?? [])
+                .Select(item => new JellyfinPathReplacementItem(item.Id, item.RemotePath, item.LocalPath))
+                .ToList());
+
+    public UpdateEmbyPathReplacements ToEmbyCommand(int sourceId) =>
+        new(
+            sourceId,
+            (Items ?? [])
+                .Select(item => new EmbyPathReplacementItem(item.Id, item.RemotePath, item.LocalPath))
+                .ToList());
+}

ErsatzTV/Controllers/Api/Requests/ReplaceRemoteLibraryPreferencesRequest.cs

@@ -0,0 +1,29 @@
+using ErsatzTV.Application.Emby;
+using ErsatzTV.Application.Jellyfin;
+using ErsatzTV.Application.Plex;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+// Shared by Plex/Jellyfin/Emby (design #202 §C4a) — the PUT body is the complete set of the
+// source's libraries; a row absent from the request is left untouched, not deleted. The controller
+// validates the id set against that source's known libraries before dispatch.
+public record ReplaceRemoteLibraryPreferencesRequest(List<RemoteLibraryPreferenceRequest> Libraries)
+{
+    public UpdatePlexLibraryPreferences ToPlexCommand() =>
+        new(
+            (Libraries ?? [])
+                .Select(item => new PlexLibraryPreference(item.Id, item.ShouldSyncItems))
+                .ToList());
+
+    public UpdateJellyfinLibraryPreferences ToJellyfinCommand() =>
+        new(
+            (Libraries ?? [])
+                .Select(item => new JellyfinLibraryPreference(item.Id, item.ShouldSyncItems))
+                .ToList());
+
+    public UpdateEmbyLibraryPreferences ToEmbyCommand() =>
+        new(
+            (Libraries ?? [])
+                .Select(item => new EmbyLibraryPreference(item.Id, item.ShouldSyncItems))
+                .ToList());
+}

ErsatzTV/Controllers/Api/Requests/SaveRemoteConnectionRequest.cs

@@ -0,0 +1,29 @@
+using ErsatzTV.Application.Emby;
+using ErsatzTV.Application.Jellyfin;
+using ErsatzTV.Core.Emby;
+using ErsatzTV.Core.Jellyfin;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+// ApiKey blank/omitted = retain the existing key (design #202 secure connection contract, finding 1)
+// — the caller passes the source's current key so ToCommand can substitute it when the incoming
+// value is blank. A non-blank value sets a new key. Required on first connect (no existing key),
+// which the controller/handler enforces before dispatch.
+public record SaveRemoteConnectionRequest(string Address, string ApiKey)
+{
+    public SaveJellyfinSecrets ToJellyfinCommand(string existingApiKey) =>
+        new(
+            new JellyfinSecrets
+            {
+                Address = Address,
+                ApiKey = string.IsNullOrWhiteSpace(ApiKey) ? existingApiKey : ApiKey
+            });
+
+    public SaveEmbySecrets ToEmbyCommand(string existingApiKey) =>
+        new(
+            new EmbySecrets
+            {
+                Address = Address,
+                ApiKey = string.IsNullOrWhiteSpace(ApiKey) ? existingApiKey : ApiKey
+            });
+}

ErsatzTV/Controllers/Api/Requests/UpdateLocalLibraryPathRequest.cs

@@ -0,0 +1,10 @@
+using ErsatzTV.Application.Libraries;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+// Id 0/absent = new path (matches UpdateLocalLibraryHandler's normalized-path-string identity merge;
+// the Id is advisory only — see design #202 §C4c).
+public record UpdateLocalLibraryPathRequest(int Id, string Path)
+{
+    public UpdateLocalLibraryPath ToCommand() => new(Id, Path);
+}

ErsatzTV/Controllers/Api/Requests/UpdateLocalLibraryRequest.cs

@@ -0,0 +1,16 @@
+using ErsatzTV.Application.Libraries;
+
+namespace ErsatzTV.Controllers.Api.Requests;
+
+// MediaKind is immutable after create, so it is deliberately absent here (structural immutability
+// instead of a 422 — see design #202 §C3).
+public record UpdateLocalLibraryRequest(string Name, List<UpdateLocalLibraryPathRequest> Paths)
+{
+    public UpdateLocalLibrary ToCommand(int id) =>
+        new(
+            id,
+            Name,
+            (Paths ?? [])
+                .Select(path => path.ToCommand())
+                .ToList());
+}