fix(crawler): budget image tail against actual sniff-prefix length

The per-image download cap subtracted the constant SNIFF_PREFIX_BYTES (64)
from the budget instead of the bytes actually drained into the prefix.
Because the prefix loop appends whole chunks, a single ~16 KiB first chunk
fills the 64-byte sniff window in one drain, so the tail could then add
another near-full cap — storing up to ~2× max_image_bytes per page.

Capture prefix.len() before the prefix is moved into the stream and budget
the tail as `max_image_bytes - prefix_len` via a small `remaining_after_prefix`
helper (unit-tested for the overshoot and saturation cases).

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 = "0.93.3"
+version = "0.93.4"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.93.3"
+version = "0.93.4"
 edition = "2021"
 default-run = "mangalord"
 

backend/src/crawler/content.rs

@@ -421,6 +421,15 @@ pub(crate) struct StoredPage {
 /// "first 16 bytes" diagnostic in the error path is useful.
 const SNIFF_PREFIX_BYTES: usize = 64;
 
+/// Bytes still admissible for the streaming tail after the sniff prefix
+/// has been drained. The prefix already counts against the per-image cap,
+/// so the tail budget is `max_image_bytes - prefix_len` using the
+/// **actual** drained length — never a constant — so `prefix_len + tail`
+/// can never exceed the cap.
+fn remaining_after_prefix(max_image_bytes: usize, prefix_len: usize) -> usize {
+    max_image_bytes.saturating_sub(prefix_len)
+}
+
 /// Download a single page image, validate it's really an image, and
 /// stream it to storage. Returns the storage key + content type. Does
 /// not touch the DB — persistence is batched into one short transaction
@@ -494,11 +503,18 @@ async fn download_and_store_page(
     // straight to storage. The cap is enforced via a running total in
     // the stream adapter so a server that omits Content-Length still
     // can't exhaust memory.
+    // Budget the streaming tail against the bytes *actually* drained into
+    // the prefix, not the constant `SNIFF_PREFIX_BYTES`. The prefix loop
+    // appends whole chunks, so a single 16 KiB first chunk fills the
+    // 64-byte sniff window in one drain — charging only 64 bytes would
+    // then let the tail add another full `max_image_bytes`, storing up to
+    // ~2× the cap. Capture the length before `prefix` is moved into the
+    // stream below.
+    let prefix_len = prefix.len();
     let prefix_stream = futures_util::stream::once(async move {
         Ok::<bytes::Bytes, StorageError>(prefix)
     });
-    let prefix_len = SNIFF_PREFIX_BYTES.min(max_image_bytes);
-    let mut remaining = max_image_bytes.saturating_sub(prefix_len);
+    let mut remaining = remaining_after_prefix(max_image_bytes, prefix_len);
     let url_for_err = url.clone();
     let rest_stream = body.map(move |frame| match frame {
         Ok(chunk) => {
@@ -644,6 +660,30 @@ mod tests {
         assert!(guard_nav_url("http://manga-host.test/c/1/p/2").is_ok());
     }
 
+    #[test]
+    fn tail_budget_uses_actual_prefix_length_not_constant() {
+        // A single 16 KiB first chunk fills the 64-byte sniff window in one
+        // drain, so the streaming tail budget must be `cap - 16KiB`, not
+        // `cap - 64`. The constant-64 math previously admitted a further
+        // ~full cap on top of the already-drained prefix (~2× overshoot).
+        let cap = 20 * 1024;
+        let prefix_len = 16 * 1024; // one real chunk, well over SNIFF_PREFIX_BYTES
+        let remaining = remaining_after_prefix(cap, prefix_len);
+        assert_eq!(remaining, cap - prefix_len);
+        // Invariant: prefix + admitted tail never exceeds the cap.
+        assert!(prefix_len + remaining <= cap);
+        // And it's strictly tighter than the old constant-64 budget.
+        assert!(remaining < cap - SNIFF_PREFIX_BYTES);
+    }
+
+    #[test]
+    fn tail_budget_saturates_when_prefix_hits_cap() {
+        // Body shorter than the sniff window: prefix drained == cap, tail
+        // budget is zero (no negative underflow).
+        assert_eq!(remaining_after_prefix(50, 50), 0);
+        assert_eq!(remaining_after_prefix(50, 64), 0);
+    }
+
     #[tokio::test]
     async fn cleanup_orphans_deletes_written_keys() {
         let dir = tempfile::tempdir().unwrap();

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.93.3",
+  "version": "0.93.4",
   "private": true,
   "type": "module",
   "scripts": {