fix(import): capture photo dimensions in fileWatcher + s3AutoImporter (#447)

The aspect-aware gallery layouts (masonry / mosaic / justified) read
photo.width and photo.height to size each card to the source's real
proportions. Two import paths were inserting rows without those
fields, which forced MasonryGalleryLayout to fall back to a hard-coded
800×600 default — every card came out the same shape, so users
reported masonry as "always cropped to 1:1ish" no matter which
thumbnail fit mode they chose.

- fileWatcher.js: extract dims with sharp.metadata() before insert.
- s3AutoImporter.js: same, materialising a tmp local copy via
  withLocalCopy so it works in S3 mode.
- migration 090: backfill any pre-existing rows with NULL dims
  (skips videos, skips S3 deployments — those need the writer fix
  alone since migrations cannot reach the storage backend).
- imageProcessor.js: change DEFAULT_THUMBNAIL_FIT from 'cover' to
  'inside' (only kicks in when the seed setting is missing — existing
  installs keep their saved value). Add UI tooltip recommending
  'inside' for masonry/mosaic/justified, 'cover' for uniform grids.

i18n covers all six locales.

frontend/src/features/settings/tabs/ThumbnailsTab.tsx

@@ -229,6 +229,9 @@ export const ThumbnailsTab: React.FC = () => {
           <p className="text-xs text-neutral-500 dark:text-neutral-400 mt-1">
             {t('settings.thumbnails.fitHelp', 'How images are resized to fit the thumbnail dimensions. "Cover" crops to fill, "Contain" fits within bounds.')}
           </p>
+          <p className="text-xs text-neutral-500 dark:text-neutral-400 mt-2">
+            {t('settings.thumbnails.fitRecommendation', 'Recommendation: use "Inside" for masonry / mosaic / justified layouts (preserves aspect ratio); "Cover" for uniform 1:1 grid tiles.')}
+          </p>
         </div>
       </Card>