feat: apply EXIF orientation and colorspace correction to ffmpeg thumbnails

HEIC/HEIF sources use Display P3 color primaries. Without
colorspace=bt709 the mjpeg encoder treated P3 values as sRGB,
producing warm/oversaturated output. Also bake EXIF Orientation
tag into pixels so saved JPEGs are canonically oriented.

- Extract orientation from exif-reader and pass through all
  ffmpeg thumbnail paths (small, large, xlarge previews)
- Add shared build_image_thumb_filter() for the 200px path
- Add rotation + colorspace=bt709 to large/xlarge ffmpeg paths

src/thumbnails.rs

@@ -98,7 +98,7 @@ pub fn generate_image_thumbnail(src: &Path, thumb_path: &Path) -> std::io::Resul
     }
 
     if file_types::needs_ffmpeg_thumbnail(src) {
-        return generate_image_thumbnail_ffmpeg(src, thumb_path);
+        return generate_image_thumbnail_ffmpeg(src, thumb_path, orientation);
     }
 
     let img = image::open(src).map_err(|e| {
@@ -140,7 +140,7 @@ pub fn generate_large_preview(src: &Path, dest: &Path) -> std::io::Result<()> {
     }
 
     if file_types::needs_ffmpeg_thumbnail(src) {
-        return generate_large_preview_ffmpeg(src, dest);
+        return generate_large_preview_ffmpeg(src, dest, orientation);
     }
 
     let img = image::open(src).map_err(|e| {
@@ -175,14 +175,34 @@ fn encode_large_jpeg(img: image::DynamicImage, dest: &Path) -> std::io::Result<(
 /// ffmpeg path for HEIC/HEIF (image crate can't decode these). Mirrors
 /// [`crate::video::actors::generate_image_thumbnail_ffmpeg`] but scales
 /// to the large-preview cap instead of 200.
-fn generate_large_preview_ffmpeg(src: &Path, dest: &Path) -> std::io::Result<()> {
-    // scale=W:-1 with force_original_aspect_ratio=decrease + the min(iw,W)
-    // trick caps the long edge regardless of orientation, mirroring what
-    // image::thumbnail does for the non-ffmpeg branch.
-    let vf = format!(
+fn generate_large_preview_ffmpeg(
+    src: &Path,
+    dest: &Path,
+    orientation: i32,
+) -> std::io::Result<()> {
+    // Rotation + scale + colorspace. HEIC sources use Display P3; without
+    // colorspace=bt709 the mjpeg encoder treats P3 values as sRGB, producing
+    // warm/oversaturated output. The min(iw,cap) trick caps the long edge
+    // regardless of orientation, mirroring image::thumbnail.
+    let rotation = match orientation {
+        2 => "hflip",
+        3 => "transpose=2",
+        4 => "vflip",
+        5 => "transpose=0,hflip",
+        6 => "transpose=0",
+        7 => "transpose=1,hflip",
+        8 => "transpose=1",
+        _ => "",
+    };
+    let scale_expr = format!(
         "scale='if(gt(iw,ih),min(iw,{cap}),-1)':'if(gt(iw,ih),-1,min(ih,{cap}))'",
         cap = LARGE_PREVIEW_MAX_DIM
     );
+    let vf = if rotation.is_empty() {
+        format!("{},colorspace=bt709", scale_expr)
+    } else {
+        format!("{},{},colorspace=bt709", rotation, scale_expr)
+    };
     let output = Command::new("ffmpeg")
         .arg("-y")
         .arg("-i")
@@ -232,7 +252,7 @@ pub fn generate_xlarge_preview(src: &Path, dest: &Path) -> std::io::Result<()> {
     }
 
     if file_types::needs_ffmpeg_thumbnail(src) {
-        return generate_xlarge_preview_ffmpeg(src, dest);
+        return generate_xlarge_preview_ffmpeg(src, dest, orientation);
     }
 
     let img = image::open(src).map_err(|e| {
@@ -260,11 +280,30 @@ fn encode_xlarge_jpeg(img: image::DynamicImage, dest: &Path) -> std::io::Result<
     Ok(())
 }
 
-fn generate_xlarge_preview_ffmpeg(src: &Path, dest: &Path) -> std::io::Result<()> {
-    let vf = format!(
+fn generate_xlarge_preview_ffmpeg(
+    src: &Path,
+    dest: &Path,
+    orientation: i32,
+) -> std::io::Result<()> {
+    let rotation = match orientation {
+        2 => "hflip",
+        3 => "transpose=2",
+        4 => "vflip",
+        5 => "transpose=0,hflip",
+        6 => "transpose=0",
+        7 => "transpose=1,hflip",
+        8 => "transpose=1",
+        _ => "",
+    };
+    let scale_expr = format!(
         "scale='if(gt(iw,ih),min(iw,{cap}),-1)':'if(gt(iw,ih),-1,min(ih,{cap}))'",
         cap = XLARGE_PREVIEW_MAX_DIM
     );
+    let vf = if rotation.is_empty() {
+        format!("{},colorspace=bt709", scale_expr)
+    } else {
+        format!("{},{},colorspace=bt709", rotation, scale_expr)
+    };
     let output = Command::new("ffmpeg")
         .arg("-y")
         .arg("-i")