feat(pipeline): render-derived shot thumbnails, and bulk shot delete

Two independent features landing together:

Generated shot thumbnails — a shot can now carry a thumbnail promoted
from a render export (Shot.generatedThumbnailUrl/generatedThumbnailFrom)
alongside the existing brief-supplied thumbnailUrl, which always takes
display precedence (lib/shot-thumbnail.ts) so a promoted frame can only
ever fill a gap, never overwrite the brief. Shows with
pipelineConfig.autoShotThumbnail promote automatically on every
finished export (lib/render-pipeline/preview.ts); promoteExportThumbnail/
clearGeneratedThumbnail (actions/shots.ts) are the manual equivalent for
everyone else. Worker-side seek logic for picking the promoted frame is
covered by RenderWorker/VFXReviewWorker.Tests/ThumbnailSeekTests.cs.

Bulk shot delete — the shot-status page's selection bar gets a "Bulk
Actions" dropdown (alongside the existing Change Due Date) with Delete
Shots. bulkDeleteShots() (actions/shots.ts) deletes one at a time rather
than in a single statement: exports_shotId_fkey is ON DELETE RESTRICT
by design, so a shot with renders or deliveries on record is protected
and gets skipped and reported by name rather than aborting the whole
selection or being silently destroyed along with its history.

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

RenderWorker/scripts/vfxr_build_preview.jsx

@@ -30,7 +30,11 @@
         queued: [],
         frameCount: null,
         width: null,
-        height: null
+        height: null,
+        // Seconds of slate ahead of picture in the preview comp — the SHOT
+        // layer's in-point. The worker needs it to grab a thumbnail from
+        // picture rather than from the slate card.
+        pictureStartSeconds: null
     };
     var resultPath = null;
 
@@ -512,6 +516,8 @@
 
         var shotLayer = previewComp.layer("SHOT");
         var targetOutPoint = shotLayer.inPoint + shotComp.duration;
+        // Everything before this is slate; the thumbnail has to start after it.
+        result.pictureStartSeconds = shotLayer.inPoint;
         if (targetOutPoint > previewComp.duration) {
             previewComp.duration = targetOutPoint;
         }