fix(imagemagick): handle multi-page PDF outputs and improve rasterization quality

[OrbanGergo]

Summary

Fix ImageMagick multi-page PDF conversions by storing the actual generated output filenames instead of assuming a single output file.

ImageMagick generates numbered output files for multi-page PDFs (e.g. filename-0.png, filename-1.png), but only filename.png was stored in the database. As a result, the View and Download actions attempted to access a file that did not exist.

Also increase PDF rasterization density to 300 DPI to improve PDF-to-image conversion quality.

Changes

• Detect generated output files after conversion.
• Store all generated output filenames in the file_names table.
• Preserve existing behavior for single-output conversions.
• Increase PDF rasterization density to 300 DPI for improved image quality.

Validation

• Converted a 2-page PDF to PNG using ImageMagick.
• Verified that all generated output files are stored in the file_names table.
• Verified that View and Download work correctly for each generated output file.
• Verified that PDF-to-PNG output quality is improved with 300 DPI.

Closes #541
Closes #340

---

Summary by cubic

Fix multi-page PDF conversions by saving all ImageMagick outputs and improve PDF-to-image quality with 300 DPI rasterization. This restores working View/Download links per page and produces sharper images.

• Bug Fixes
  • Discover, filter, and numerically sort generated files (e.g., filename-0.png, filename-1.png) instead of assuming a single output.
  • Preserve single-file behavior when only one output is produced.
  • Add -density 300 for pdf input to improve rasterization quality.

^{Written for commit ad91743. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 3 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>