Add backlinks feature (issue #164)

[compscidr]

Summary

• Track internal backlinks between blog posts by parsing markdown content for /posts/YYYY/MM/DD/slug links
• Track external backlinks via HTTP Referer headers with hit counts and timestamps
• Extract external links from post content for admin visibility
• Display four admin-only sections on post pages: Backlinks, Linked Posts, External Links, External Backlinks
• Compute backlinks on post create/update and seed existing posts during migration

Changes

• blog/post.go: Added Backlink, ExternalBacklink models and ExternalLinks() method
• blog/blog.go: Added ComputeBacklinks, GetBacklinks, GetOutboundLinks, GetExternalBacklinks, TrackReferer methods; wired referer tracking into Post() and NoRoute() handlers
• admin/admin.go: Call ComputeBacklinks after create/update; pass backlink data to admin template
• templates/post.html: Added admin-only backlink sections
• templates/post-admin.html: Added backlink sections
• tools/migrate.go: Added new models to AutoMigrate; added seedBacklinks for existing posts
• blog/blog_test.go: Added TestBacklinks and TestExternalBacklinks tests

Test plan

• go test ./... passes
• Create two posts where one links to the other; verify backlink appears on target post
• Visit a post with an external referer header; verify external backlink recorded
• Verify external URLs in post content appear under "External Links"
• Verify non-admin users do not see backlink sections

Closes #164

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 54.96183% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.36%. Comparing base (5901b46) to head (2054e7c).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/migrate.go	31.03%	19 Missing and 1 partial ⚠️
blog/blog.go	76.25%	15 Missing and 4 partials ⚠️
blog/post.go	0.00%	11 Missing ⚠️
admin/admin.go	18.18%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #478      +/-   ##
==========================================
+ Coverage   43.23%   45.36%   +2.13%     
==========================================
  Files           6        6              
  Lines        1071     1186     +115     
==========================================
+ Hits          463      538      +75     
- Misses        573      607      +34     
- Partials       35       41       +6     

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[Copilot]

Pull request overview

Adds a backlinks feature to the blog by persisting (1) internal post-to-post link relationships derived from markdown content and (2) external backlinks derived from HTTP Referer headers, then exposing this data in admin-only UI sections.

Changes:

• Introduces Backlink and ExternalBacklink models plus post-level external link extraction.
• Adds backlink computation/retrieval + referer tracking logic to blog handlers.
• Extends migrations to create/seed backlink data and adds tests for both internal and external backlinks.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
tools/migrate.go	Migrates new tables and seeds internal backlinks for existing posts.
blog/post.go	Adds backlink models and Post.ExternalLinks() extraction helper.
blog/blog.go	Implements backlink computation/queries and referer tracking; wires tracking into handlers.
admin/admin.go	Triggers backlink recomputation on post create/update; passes backlink data to templates.
templates/post.html	Adds admin-only sections for backlinks/outbound links/external links/external backlinks.
templates/post-admin.html	Adds backlink/outbound/external link sections to the admin post view.
blog/blog_test.go	Adds tests covering internal backlinks and external referer tracking behavior.

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