image: add xlarge (4096px) on-demand preview tier

New `PhotoSize::XLarge` variant sits between `Large` (2048px) and
`Full` (original). On-demand generated and disk-cached at
`_xlarge/<hash>.jpg`, same waterfall as `Large` (embedded RAW preview
→ ffmpeg → image crate). Sources below 4096px serve at native size.

Reduces decoded bitmap memory from ~192MB (48MP full) to ~64MB for
the mobile viewer's zoom tier.

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

src/thumbnails.rs

@@ -36,12 +36,19 @@ use crate::video::actors::{generate_image_thumbnail_ffmpeg, generate_video_thumb
 /// `size=full` and the handler streams the original bytes.
 pub const LARGE_PREVIEW_MAX_DIM: u32 = 2048;
 
-/// JPEG quality for the large preview tier. 85 is the conventional
-/// "indistinguishable from source at viewing size" point — well above the
-/// `image` crate's default ~75, but well below quality-90+ territory where
-/// file size doubles for no perceptible win.
+/// JPEG quality for the large and xlarge preview tiers. 85 is the
+/// conventional "indistinguishable from source at viewing size" point —
+/// well above the `image` crate's default ~75, but well below quality-90+
+/// territory where file size doubles for no perceptible win.
 const LARGE_PREVIEW_JPEG_QUALITY: u8 = 85;
 
+/// Maximum long-edge size (px) for the xlarge preview tier. Bridges the
+/// gap between `large` (2048px, ~16MB decoded) and the original bytes
+/// (potentially 48+ MP / ~192MB decoded). At 4096px the decoded bitmap is
+/// ~64MB — enough for 2-3× pinch-zoom on any phone before the viewer
+/// needs to stream the true original.
+pub const XLARGE_PREVIEW_MAX_DIM: u32 = 4096;
+
 lazy_static! {
     pub static ref IMAGE_GAUGE: IntGauge = IntGauge::new(
         "imageserver_image_total",
@@ -205,6 +212,86 @@ fn generate_large_preview_ffmpeg(src: &Path, dest: &Path) -> std::io::Result<()>
     Ok(())
 }
 
+/// Generate the on-demand xlarge-preview tier (≈4096 long edge JPEG).
+///
+/// Same waterfall as [`generate_large_preview`] but targeting
+/// [`XLARGE_PREVIEW_MAX_DIM`]. Sources whose long edge is already below
+/// the cap are encoded at native size (no upscale).
+pub fn generate_xlarge_preview(src: &Path, dest: &Path) -> std::io::Result<()> {
+    let orientation = exif::read_orientation(src).unwrap_or(1);
+
+    if let Some(preview) = exif::extract_embedded_jpeg_preview(src) {
+        let img = image::load_from_memory(&preview).map_err(|e| {
+            std::io::Error::new(
+                std::io::ErrorKind::InvalidData,
+                format!("decode embedded preview {:?}: {}", src, e),
+            )
+        })?;
+        let img = exif::apply_orientation(img, orientation);
+        return encode_xlarge_jpeg(img, dest);
+    }
+
+    if file_types::needs_ffmpeg_thumbnail(src) {
+        return generate_xlarge_preview_ffmpeg(src, dest);
+    }
+
+    let img = image::open(src).map_err(|e| {
+        std::io::Error::new(std::io::ErrorKind::InvalidData, format!("{:?}: {}", src, e))
+    })?;
+    let img = exif::apply_orientation(img, orientation);
+    encode_xlarge_jpeg(img, dest)
+}
+
+fn encode_xlarge_jpeg(img: image::DynamicImage, dest: &Path) -> std::io::Result<()> {
+    let (w, h) = img.dimensions();
+    let max_dim = w.max(h);
+    let scaled = if max_dim > XLARGE_PREVIEW_MAX_DIM {
+        img.thumbnail(XLARGE_PREVIEW_MAX_DIM, XLARGE_PREVIEW_MAX_DIM)
+    } else {
+        img
+    };
+    let file = std::fs::File::create(dest)
+        .map_err(|e| std::io::Error::other(format!("create {:?}: {}", dest, e)))?;
+    let mut writer = std::io::BufWriter::new(file);
+    let mut encoder = JpegEncoder::new_with_quality(&mut writer, LARGE_PREVIEW_JPEG_QUALITY);
+    encoder
+        .encode_image(&scaled)
+        .map_err(|e| std::io::Error::other(format!("encode {:?}: {}", dest, e)))?;
+    Ok(())
+}
+
+fn generate_xlarge_preview_ffmpeg(src: &Path, dest: &Path) -> std::io::Result<()> {
+    let vf = format!(
+        "scale='if(gt(iw,ih),min(iw,{cap}),-1)':'if(gt(iw,ih),-1,min(ih,{cap}))'",
+        cap = XLARGE_PREVIEW_MAX_DIM
+    );
+    let output = Command::new("ffmpeg")
+        .arg("-y")
+        .arg("-i")
+        .arg(src)
+        .arg("-vframes")
+        .arg("1")
+        .arg("-vf")
+        .arg(&vf)
+        .arg("-q:v")
+        .arg("5")
+        .arg("-f")
+        .arg("image2")
+        .arg("-c:v")
+        .arg("mjpeg")
+        .arg(dest)
+        .output()?;
+
+    if !output.status.success() {
+        return Err(std::io::Error::other(format!(
+            "ffmpeg failed ({}): {}",
+            output.status,
+            String::from_utf8_lossy(&output.stderr).trim()
+        )));
+    }
+    Ok(())
+}
+
 pub fn create_thumbnails(libs: &[libraries::Library], excluded_dirs: &[String]) {
     let tracer = global_tracer();
     let span = tracer.start("creating thumbnails");