feat(analysis): slice tall pages to the pixel budget for reliable OCR

Very tall webtoon pages were squashed by the fixed longest-edge downscale,
so OCR was garbage. The vision client now slices to the model's pixel
budget at native resolution and assembles one result:

- plan_slices: native-resolution bands sized to max_pixels (portrait OR
  landscape), only when height/width > tall_aspect_threshold; min_slice_
  height guards pathologically wide pages (reduce width); max_slices caps
  the count (coarser fallback). Normal pages still take one combined call.
- Two-pass, OCR-first: Pass A OCRs each band (near-native), merge_ocr
  stitches them with seam-scoped fuzzy dedup (keep the longer transcript,
  preserve order/kind, don't collapse non-adjacent repeats); Pass B feeds
  the whole downscaled image + the merged OCR text to get tags/scene/safety
  grounded in the real dialogue. Only OCR is merged.
- Config: replace max_image_dim with max_pixels / min_slice_height /
  slice_overlap / tall_aspect_threshold / max_slices (+ env_f64); bump
  job_timeout default 180->600 (N sequential slice calls per page); raise
  MAX_OCR_PIECES 60->200. New prompts/schemas: OCR_PROMPT/ocr_json_schema,
  GROUNDING_PROMPT/grounding_json_schema.

Tests: plan_slices (single/portrait/landscape/pathological-wide/cap +
coverage/overlap), merge_ocr (seam dedup, keep-longer, non-adjacent
repeats, order), render_whole/render_slice, the new request builders, and
updated config defaults/env.

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

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.77.1"
+version = "0.78.0"
 dependencies = [
  "anyhow",
  "argon2",