feat(presets): pre-fill the user's project from preset values (A4)

"Use this example" now actually fills the new project, not just stores a ref.

- studio-svc: CreateProjectAsync applies the chosen preset story's saved values
  after the template-graph copy. ApplyPresetValuesAsync reads
  content.preset_stories.scenes_spa = { values: {contentKey:value},
  colors: {elementKey:hex} } and overlays them onto studio.saved_scene_contents
  (by key) + saved_shared_colors/saved_scene_colors (by element_key, is_selected).
  Keys are globally unique (AE convention) so key-only matching is safe.
  Malformed scenes_spa is skipped (defaults kept). Runs in the create tx.
- admin UI: ProjectPresetStories raw scenes_spa textarea replaced with a
  structured PresetValueEditor — loads each preset scene's content elements +
  the project's shared colours and renders a type-aware input per item
  (text/textarea/number, media→upload, fill/color→colour). Serializes to
  scenes_spa {values,colors}; parses it back on edit.

Verified e2e: authored a preset with values+colour → used it → the new
project's saved_scene_contents + saved_shared_colors carry the preset values
(which the B2 render binder then writes into AE).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/studio/FlatRender.StudioSvc/Application/Services/StudioService.cs

@@ -1,3 +1,4 @@
+using System.Text.Json;
 using FlatRender.StudioSvc.Domain.Entities;
 using FlatRender.StudioSvc.Domain.Enums;
 using FlatRender.StudioSvc.Infrastructure.Data;
@@ -91,10 +92,67 @@ public class StudioService(StudioDbContext db)
         if (req.CopyDefaultValues && req.OriginalProjectId != Guid.Empty)
             await CopyTemplateGraphAsync(project.Id, req.OriginalProjectId);
 
+        // Pre-fill from the chosen preset story (premade example video): overlay its
+        // saved values onto the freshly-copied scene contents + colours.
+        if (req.PresetStoryId.HasValue)
+            await ApplyPresetValuesAsync(project.Id, req.PresetStoryId.Value);
+
         await tx.CommitAsync();
         return await GetProjectAsync(project.Id, userId);
     }
 
+    /// <summary>
+    /// Applies an admin-authored preset story's filled values onto a freshly-created
+    /// project. The preset stores them in content.preset_stories.scenes_spa as
+    /// <c>{ "values": { contentKey: value }, "colors": { elementKey: hex } }</c>.
+    /// Content/colour keys are globally unique (AE naming convention), so matching by
+    /// key alone reaches the right scene element. Runs inside the create transaction.
+    /// </summary>
+    private async Task ApplyPresetValuesAsync(Guid savedProjectId, Guid presetStoryId)
+    {
+        var spa = await db.Database
+            .SqlQuery<string?>($@"SELECT scenes_spa AS ""Value"" FROM content.preset_stories
+                                  WHERE id = {presetStoryId} AND deleted_at IS NULL")
+            .FirstOrDefaultAsync();
+        if (string.IsNullOrWhiteSpace(spa)) return;
+
+        JsonElement root;
+        try { root = JsonDocument.Parse(spa).RootElement; }
+        catch { return; } // malformed scenes_spa → skip pre-fill, leave defaults
+        if (root.ValueKind != JsonValueKind.Object) return;
+
+        // content element values → saved_scene_contents.value (match by key)
+        if (root.TryGetProperty("values", out var vals) && vals.ValueKind == JsonValueKind.Object)
+        {
+            var valuesJson = vals.GetRawText();
+            await db.Database.ExecuteSqlInterpolatedAsync($@"
+                UPDATE studio.saved_scene_contents c
+                SET value = pv.value, updated_at = now()
+                FROM json_each_text({valuesJson}::json) AS pv(key, value)
+                WHERE c.key = pv.key
+                  AND c.saved_scene_id IN (
+                      SELECT id FROM studio.saved_scenes WHERE saved_project_id = {savedProjectId});");
+        }
+
+        // colour values → shared + per-scene colours (match by element_key)
+        if (root.TryGetProperty("colors", out var cols) && cols.ValueKind == JsonValueKind.Object)
+        {
+            var colorsJson = cols.GetRawText();
+            await db.Database.ExecuteSqlInterpolatedAsync($@"
+                UPDATE studio.saved_shared_colors sc
+                SET value = pc.value, is_selected = true
+                FROM json_each_text({colorsJson}::json) AS pc(key, value)
+                WHERE sc.element_key = pc.key AND sc.saved_project_id = {savedProjectId};");
+            await db.Database.ExecuteSqlInterpolatedAsync($@"
+                UPDATE studio.saved_scene_colors c
+                SET value = pc.value, is_selected = true
+                FROM json_each_text({colorsJson}::json) AS pc(key, value)
+                WHERE c.element_key = pc.key
+                  AND c.saved_scene_id IN (
+                      SELECT id FROM studio.saved_scenes WHERE saved_project_id = {savedProjectId});");
+        }
+    }
+
     /// <summary>
     /// Copies a content template project's scenes, content elements and colour elements
     /// into a freshly-created editable project. Runs inside the caller's transaction;