fix(crawler): cap the number of page images per chapter

The per-image byte cap bounds each download but not the count, so a
hostile or compromised reader page listing thousands of <img> tags could
drive an unbounded disk fill. Add `CRAWLER_MAX_IMAGES_PER_CHAPTER`
(CrawlerConfig::max_images_per_chapter, default 2000, 0 = disabled) and
reject an over-cap chapter with a failed ack (exponential backoff) rather
than downloading it.

Threaded through sync_chapter_content and its three call sites (daemon
dispatcher, admin resync, CLI); enforced via `image_count_over_cap` right
after parse. The cap is an env-only safety knob, preserved across settings
reloads.

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

backend/src/settings.rs

@@ -254,6 +254,9 @@ impl CrawlerSettings {
                 .map(str::to_string),
             download_allowlist,
             max_image_bytes: self.max_image_bytes as usize,
+            // Env-only safety cap, not a runtime-editable setting — preserve
+            // it from the base so a settings reload keeps the boot value.
+            max_images_per_chapter: base.max_images_per_chapter,
             manga_limit: self.manga_limit as usize,
             job_timeout: Duration::from_secs(self.job_timeout_secs.max(1)),
             metadata_max_consecutive_failures: self.metadata_max_consecutive_failures,