perf: denormalize media_sort_date + pre-scale video thumbs

Schema (media_sort_date denormalization):
- Add media_sort_date column to storage.files with DEFAULT created_at
- Add trigger sync_media_sort_date: when file_metadata is upserted,
  copies COALESCE(captured_at, created_at) into files.media_sort_date
- Add partial index idx_files_media_timeline on (user_id, media_sort_date DESC)
  WHERE NOT is_trashed AND media type -- enables Index Scan + Limit (no Sort)
- copy_folder_tree now copies media_sort_date for copied files
- Remove dead idx_file_metadata_captured (no longer needed)

Query optimization (list_media_files):
- Rewrite to use fi.media_sort_date instead of COALESCE(fm.captured_at,...)
- Eliminates LEFT JOIN file_metadata -- one fewer table touch
- Plan: Limit to Index Scan O(LIMIT) instead of Sort O(N)

Video thumbnail pre-scaling (client + server):
- JS: pre-scale canvas to max 400px before toBlob -- 22x less RAM, 15x less BW
- Rust: fast-path in store_external_thumbnail -- if payload is already
  WebP with dims within max_dim, store as-is (zero decode, zero encode)

static/js/features/library/photos.js

@@ -257,11 +257,16 @@ const photosView = {
         }, { once: true });
 
         video.addEventListener('seeked', () => {
+            // Pre-scale to thumbnail size in the browser — saves ~22× RAM,
+            // ~15× bandwidth, and lets the server skip resize entirely.
+            const MAX_THUMB = 400; // must match ThumbnailSize::Preview
+            const scale = Math.min(MAX_THUMB / video.videoWidth,
+                                   MAX_THUMB / video.videoHeight, 1);
             const canvas = document.createElement('canvas');
-            canvas.width = video.videoWidth;
-            canvas.height = video.videoHeight;
+            canvas.width  = Math.round(video.videoWidth  * scale);
+            canvas.height = Math.round(video.videoHeight * scale);
             const ctx = canvas.getContext('2d');
-            ctx.drawImage(video, 0, 0);
+            ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
 
             // Try WebP first, fall back to JPEG
             const mimeType = typeof canvas.toBlob === 'function'