fix(analysis): cap decoded pixels on the vision backend too

The vision prep pass decoded pages with bare `image::load_from_memory`,
which applies no allocation bound. `max_pixels` only downscales after a
full decode, so a tiny WebP/JPEG header declaring huge dimensions could
OOM the blocking worker — the same decompression-bomb the OCR backend
already guards against. Manga pages are commonly WebP/JPEG, where the
format's own self-limits are weaker than PNG's.

Route the decode through `decode_within`, applying an `image::Limits`
alloc cap sized from the shared `ocr_max_decode_pixels`
(ANALYSIS_OCR_MAX_DECODE_PIXELS). Add real-WebP bomb coverage asserting
both the helper and the end-to-end Undecodable fallback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.93.6"
+version = "0.93.7"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.93.6"
+version = "0.93.7"
 edition = "2021"
 default-run = "mangalord"
 

backend/src/analysis/vision.rs

@@ -55,6 +55,12 @@ struct SliceParams {
     overlap: f64,
     tall_threshold: f64,
     max_slices: usize,
+    /// Hard cap on the **decoded** pixel count, enforced as an allocation
+    /// limit on the decoder itself. `max_pixels` only downscales *after* a
+    /// full decode, so without this a tiny WebP/JPEG/PNG declaring huge
+    /// dimensions would OOM the blocking worker (decompression bomb).
+    /// Shared with the OCR backend's cap (`ANALYSIS_OCR_MAX_DECODE_PIXELS`).
+    max_decode_pixels: u64,
 }
 
 /// Result of the (blocking-pool) prep pass: a self-contained set of byte
@@ -86,11 +92,30 @@ enum PreparedAnalysis {
 /// JPEG-encoder regression indistinguishable from "the page was just
 /// garbage." Emit a `warn` on each fallback so an operator can grep
 /// for "vision prep fell through" and tell the two apart.
+/// Decode an encoded page image with a hard allocation cap so a
+/// decompression bomb — a tiny WebP/JPEG/PNG header declaring enormous
+/// dimensions — can't OOM the blocking worker before we get a chance to
+/// downscale. The `image` crate's default reader applies no such bound;
+/// format-specific self-limits (strongest for PNG) don't cover WebP/JPEG,
+/// which manga pages commonly use. `4 bytes/px` (RGBA) leaves headroom
+/// over any single intermediate the decoder allocates per pixel. Mirrors
+/// `ocr::decode_rgb8_within`.
+fn decode_within(image: &[u8], max_decode_pixels: u64) -> Option<DynamicImage> {
+    use std::io::Cursor;
+    let mut reader = image::ImageReader::new(Cursor::new(image))
+        .with_guessed_format()
+        .ok()?;
+    let mut limits = image::Limits::default();
+    limits.max_alloc = Some(max_decode_pixels.saturating_mul(4));
+    reader.limits(limits);
+    reader.decode().ok()
+}
+
 fn prepare_analysis(image: &[u8], params: SliceParams) -> PreparedAnalysis {
-    let Some(img) = image::load_from_memory(image).ok() else {
+    let Some(img) = decode_within(image, params.max_decode_pixels) else {
         tracing::warn!(
             bytes = image.len(),
-            "vision prep fell through to Undecodable: image::load_from_memory failed"
+            "vision prep fell through to Undecodable: decode failed or exceeded pixel cap"
         );
         return PreparedAnalysis::Undecodable;
     };
@@ -162,6 +187,7 @@ impl VisionClient {
                 overlap: cfg.slice_overlap,
                 tall_threshold: cfg.tall_aspect_threshold,
                 max_slices: cfg.max_slices,
+                max_decode_pixels: cfg.ocr_max_decode_pixels,
             },
         }
     }
@@ -779,9 +805,40 @@ mod tests {
             overlap: 0.12,
             tall_threshold: 1.6,
             max_slices: 16,
+            max_decode_pixels: 100_000_000,
         }
     }
 
+    fn encode_webp(w: u32, h: u32) -> Vec<u8> {
+        use image::{DynamicImage, ImageFormat, RgbImage};
+        let img = DynamicImage::ImageRgb8(RgbImage::new(w, h));
+        let mut buf = std::io::Cursor::new(Vec::new());
+        img.write_to(&mut buf, ImageFormat::WebP).expect("encode webp");
+        buf.into_inner()
+    }
+
+    #[test]
+    fn decode_within_rejects_oversize_webp() {
+        // Non-PNG decompression-bomb coverage: a real 2000×2000 WebP (4 MP)
+        // must be refused when the decode cap is set below its pixel count.
+        // The allocation limit trips inside the decoder rather than the
+        // full frame being materialized. Manga pages are commonly WebP/JPEG,
+        // where the format's own self-limits are weaker than PNG's.
+        let webp = encode_webp(2000, 2000);
+        assert!(decode_within(&webp, 1_000).is_none(), "cap below pixels must reject");
+        // A generous cap decodes the same image fine.
+        assert!(decode_within(&webp, 100_000_000).is_some(), "within cap must decode");
+    }
+
+    #[test]
+    fn prepare_analysis_falls_back_to_undecodable_on_decode_bomb() {
+        // End-to-end: an over-cap image drives the prep pass to the
+        // Undecodable fallback (raw-bytes path) instead of decoding it.
+        let webp = encode_webp(2000, 2000);
+        let p = SliceParams { max_decode_pixels: 1_000, ..params() };
+        assert!(matches!(prepare_analysis(&webp, p), PreparedAnalysis::Undecodable));
+    }
+
     fn ocr(text: &str, kind: &str) -> OcrResult {
         OcrResult {
             text: text.into(),
@@ -1136,6 +1193,7 @@ mod tests {
             overlap: 0.05,
             tall_threshold: 1.8,
             max_slices: 6,
+            max_decode_pixels: 100_000_000,
         }
     }