feat(uploads): DNG / camera-RAW support via embedded-preview extraction (#821)

Sharp's bundled libvips has no raw loader, so a DNG can't be thumbnailed
directly. This adds a preview-extraction step so RAW/DNG uploads get a proper
thumbnail + gallery preview while the original RAW is kept for download.

- imageProcessor: isRawFilename() + extractRawPreview() (exiftool extracts the
  embedded full-res JPEG — JpgFromRaw → PreviewImage → ThumbnailImage, validated
  with sharp) + withProcessableImage() which is a pass-through for ordinary
  images and swaps in the extracted JPEG for RAW. Wired into ingest
  (photoProcessor) and all three on-demand generators (ensureThumbnail/Hero/
  Preview). generateHeroImage/generatePreviewImage gained outputBasename so
  RAW-derived outputs stay named after the source.
- Dockerfile: add exiftool (confirmed present in Alpine v3.24 community).
- Format maps: dng → image/x-adobe-dng in uploadSettings.js and fileTypes.ts;
  ALLOWED_MEDIA_TYPES gains a DNG entry (TIFF magic numbers) so it passes the
  security file-validator.

Strictly gated by extension: nothing in this path runs for jpg/png/webp/etc, so
existing photos are unaffected. If extraction fails (corrupt RAW, no embedded
preview), the photo is marked 'failed' with a clear error — same as any
unreadable upload.

Verification boundary (please validate on a real DNG after the image rebuilds):
the exiftool extraction itself couldn't be exercised in the dev sandbox
(exiftool isn't a dev dependency and there's no DNG fixture). Unit tests cover
the gating (RAW detection + non-RAW pass-through + clean failure without
exiftool); existing processPhoto tests still pass. Known limitation: a DNG is
only accepted when the browser reports its MIME as image/x-adobe-dng (Chrome
does); browsers that send an empty type reject it client- and server-side —
a follow-up can add extension-based acceptance for the RAW set.

Companion to the HEIC/dynamic-hint PR; targets main only.

backend/Dockerfile

@@ -67,8 +67,11 @@ RUN npm install -g npm@11
 # malicious) PDF. pdftoppm does not execute embedded JS or fetch remote
 # resources, so it doubles as the SSRF/phone-home guard for untrusted inbound
 # documents (see docs/accounting-inbound-invoices.md).
+# exiftool extracts the embedded full-res JPEG preview from RAW/DNG uploads
+# (Apple ProRAW etc.) — sharp's libvips has no raw loader, so the pipeline
+# thumbnails/displays that preview while keeping the original for download.
 RUN apk add --no-cache dumb-init postgresql-client ffmpeg su-exec \
-    fontconfig ttf-dejavu ttf-liberation poppler-utils && \
+    fontconfig ttf-dejavu ttf-liberation poppler-utils exiftool && \
     fc-cache -f
 
 # Create non-root user