Fix tag duplication when saving posts

[compscidr]

Summary

• Replace Association("Tags").Clear() + struct-based Updates() with Association("Tags").Replace() to atomically swap tag associations without duplication
• Remove Tags from the struct passed to Updates() so GORM doesn't re-insert them
• Add missing Tag migration to admin test setup
• Add test that saves a post with tags twice and verifies the count stays at 2

Closes #487

Test plan

• Existing tests pass
• New test verifies tags don't double on repeated saves
• Manual: edit a post with tags, save multiple times, confirm tag count stays stable

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.65%. Comparing base (ef0a4cd) to head (a68fc44).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #496      +/-   ##
==========================================
- Coverage   45.68%   45.65%   -0.03%     
==========================================
  Files           8        8              
  Lines        2038     2037       -1     
==========================================
- Hits          931      930       -1     
  Misses        990      990              
  Partials      117      117              

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[Copilot]

Pull request overview

Fixes tag duplication when repeatedly saving posts by changing how GORM tag associations are updated, and adds regression coverage in admin tests (Issue #487).

Changes:

• Switch post tag updates from Association("Tags").Clear() + struct-based Updates() to Association("Tags").Replace() to avoid duplicating associations.
• Avoid passing Tags via struct Updates() so GORM doesn’t re-insert associations.
• Update admin test DB migrations and add a regression test to ensure tag count remains stable across repeated saves.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
admin/admin.go	Updates tag persistence logic in UpdatePost to use association replacement.
admin/admin_test.go	Adds Tag migration to test setup and a regression test for repeated tag saves.

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