RAW: try IFD0 + IFD1 for embedded preview, serve at full size

The thumbnail pipeline's embedded-JPEG extractor only checked IFD1
(THUMBNAIL), which on many Nikon NEFs is missing or zero-length even
when IFD0 (PRIMARY) carries a perfectly good 1-2 MP reduced-resolution
preview the camera writes for in-body review. The previous behavior
produced black thumbs on disk: the buggy IFD1 pointer resolved to a
short byte sequence that happened to satisfy the SOI sanity check,
image::load_from_memory accepted it, and the resize path quietly wrote
a black JPEG.

Now both IFDs are checked and the larger valid JPEG wins. Format-
agnostic: applies to every TIFF-based RAW (NEF / ARW / CR2 / DNG / RAF /
ORF / RW2 / PEF / SRW / TIFF). is_tiff_raw is now pub so main.rs can
gate its full-size handler on it.

Also extends the /image handler so size=full requests for RAW formats
serve the embedded preview as image/jpeg instead of NamedFile-streaming
the original RAW bytes - browsers can't decode a .nef container, so
<img src=...> would otherwise land as a broken image. Falls through to
NamedFile if no preview is present, preserving the historical behavior
for callers that genuinely want the original bytes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -215,6 +215,23 @@ async fn get_image(
             }
         }
 
+        // Full-size requests for RAW formats (NEF/CR2/ARW/etc.) can't just
+        // NamedFile-stream the original bytes — browsers won't decode the
+        // RAW container, so a `<img src=...>` lands as a broken image. Serve
+        // the embedded JPEG preview instead (typically the camera's in-body
+        // review JPEG, ~1–2 MP). Falls through to NamedFile if no preview is
+        // available, which preserves the historical behavior for callers
+        // that genuinely want the original bytes.
+        if image_size == PhotoSize::Full && exif::is_tiff_raw(&path) {
+            if let Some(preview) = exif::extract_embedded_jpeg_preview(&path) {
+                span.set_status(Status::Ok);
+                return HttpResponse::Ok()
+                    .content_type("image/jpeg")
+                    .insert_header(("Cache-Control", "public, max-age=3600"))
+                    .body(preview);
+            }
+        }
+
         if let Ok(file) = NamedFile::open(&path) {
             span.set_status(Status::Ok);
             // Enable ETag and set cache headers for full images (1 hour cache)