docs: clarify that MAX_PAGES_PER_CHAPTER=0 relies on the body-size backstop

Make explicit (config.rs + .env.example) that disabling the per-chapter page cap
leaves MAX_REQUEST_BYTES as the sole bound (audit footgun). No behavior change;
the thumbnail-source read is already bounded by MAX_FILE_BYTES + decode limits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/config.rs

@@ -66,8 +66,10 @@ pub struct UploadConfig {
     pub max_file_bytes: usize,
     /// Max page images accepted in one chapter upload. Bounds how many
     /// parts the handler will stage before giving up, so a client can't
-    /// pin a worker streaming an unbounded page count. `0` disables the
-    /// cap. Defaults to 2000. `MAX_PAGES_PER_CHAPTER`.
+    /// pin a worker streaming an unbounded page count. `0` disables THIS
+    /// cap — the total is then bounded only by `max_request_bytes` (the
+    /// whole-request body limit), which stays the backstop either way.
+    /// Defaults to 2000. `MAX_PAGES_PER_CHAPTER`.
     pub max_pages_per_chapter: usize,
 }