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.

backend/src/services/imageProcessor.js

@@ -15,7 +15,13 @@ sharp.concurrency(2); // Limit concurrent operations
 // Default thumbnail settings
 const DEFAULT_THUMBNAIL_WIDTH = 300;
 const DEFAULT_THUMBNAIL_HEIGHT = 300;
-const DEFAULT_THUMBNAIL_FIT = 'cover'; // 'cover' for square crops
+// 'inside' preserves the source aspect ratio (output ≤ width × height).
+// This is the right default for masonry / mosaic / justified layouts —
+// the gallery sizes each card from photo.width/height and renders the
+// thumbnail with object-cover, so a thumb that already matches the
+// source aspect doesn't get re-cropped (#447). Admins who want
+// uniform 1:1 grid tiles can switch to 'cover' in the thumbnail settings.
+const DEFAULT_THUMBNAIL_FIT = 'inside';
 const DEFAULT_THUMBNAIL_QUALITY = 85;
 const DEFAULT_THUMBNAIL_FORMAT = 'jpeg';