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)

src/infrastructure/repositories/pg/file_blob_read_repository.rs

@@ -165,6 +165,12 @@ impl FileBlobReadRepository {
     ///
     /// Returns `(Vec<File>, Vec<i64>)` where the second vec contains the
     /// `sort_date` epoch for each file (used as pagination cursor).
+    ///
+    /// Uses the denormalised `media_sort_date` column (synced from
+    /// `file_metadata.captured_at` by trigger) so no JOIN with
+    /// `file_metadata` is needed.  The partial index
+    /// `idx_files_media_timeline` covers the full query: filter + ORDER BY
+    /// in a single Index Scan — O(LIMIT) not O(N).
     pub async fn list_media_files(
         &self,
         owner_id: Uuid,
@@ -178,16 +184,15 @@ impl FileBlobReadRepository {
                    EXTRACT(EPOCH FROM fi.created_at)::bigint,
                    EXTRACT(EPOCH FROM fi.updated_at)::bigint,
                    fi.user_id,
-                   EXTRACT(EPOCH FROM COALESCE(fm.captured_at, fi.created_at))::bigint AS sort_date
+                   EXTRACT(EPOCH FROM fi.media_sort_date)::bigint AS sort_date
               FROM storage.files fi
               LEFT JOIN storage.folders fo ON fo.id = fi.folder_id
-              LEFT JOIN storage.file_metadata fm ON fm.file_id = fi.id
              WHERE fi.user_id = $1
                AND NOT fi.is_trashed
                AND (fi.mime_type LIKE 'image/%' OR fi.mime_type LIKE 'video/%')
                AND ($2::bigint IS NULL
-                    OR EXTRACT(EPOCH FROM COALESCE(fm.captured_at, fi.created_at))::bigint < $2::bigint)
-             ORDER BY COALESCE(fm.captured_at, fi.created_at) DESC
+                    OR EXTRACT(EPOCH FROM fi.media_sort_date)::bigint < $2::bigint)
+             ORDER BY fi.media_sort_date DESC
              LIMIT $3
             "#,
         )