feat: traditional comic covers, animal consistency, back-cover blurb, PDF output

Cover/back improvements:
- Front cover: MANDATORY TITLE instruction makes 'BULGARIAN VOCABULARY ADVENTURE'
  dominate the top 20% of the cover in bold masthead lettering
- Back cover: Gemini generates a 2-3 sentence English marketing blurb that is
  embedded verbatim in the blurb-box instruction (italic display type)
- Both covers now specify Silver-Age / Bronze-Age comic production layout
  (price box, barcode strip, cover lines)

Animal consistency:
- Character bible prompt now treats animals as named characters — specifies exact
  breed, fur colour/pattern, markings, and eye colour; instructs model never to
  substitute a generic animal or change markings between pages
- Story page prompts add explicit animal consistency rule alongside human rules

Bible/blurb reliability:
- Switch helper calls to same SystemInstruction + user-content pattern that the
  story generator uses successfully with gemini-2.5-flash
- Increase MaxOutputTokens to 8192 and timeout to 90s to match story generator
- Retry pause extended to 15s for free-tier RPM recovery between rapid calls
- Removed deprecated gemini-2.0-flash model references

PDF generation (new internal/story/pdf.go):
- AssembleComicPDF() combines all 5 images into comic.pdf via ImageMagick convert
- Pages are in reading order: cover → story pages → back cover
- Non-fatal: warns if ImageMagick is not installed, never blocks the pipeline

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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