FOLLOW-UP: Consider replacing archived disintegration/imaging dependency

[adnaan]

Context

This follow-up task was identified during the review of PR #267.

Source PR: #267
PR Title: feat: add file upload & storage support (Roadmap 2.2)
Suggested by: @claude[bot]

Task Description

The disintegration/imaging library (v1.6.2) used by pkg/imaging is archived on GitHub (read-only, no new releases). While it works correctly today, this is a long-term maintenance concern.

Alternatives to evaluate:

• golang.org/x/image/draw — official Go image processing, actively maintained
• stdlib image/* packages — for basic resize operations without external deps
• anthonynsimon/bild — active pure-Go image processing library

The migration would only affect pkg/imaging/thumbnail.go (single call to imaging.Fit + imaging.Encode).

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This issue was automatically created by prmonitor from PR review comments.