fix(Code References): Code references are slow to query

[emyller]

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Changes

Closes #5932

Improve code references so they're usable again.

The previous data model did not handle accumulated code references too well, and we had to disable it.

This patch will:

• Complete redesign the data model of code references.
• Migrate old data to new data.
• Improve code references querying performance from [old request time p90] to [new request time p90] in staging.
• Prevent duplicate scans — for hygiene: performance gain does not rely on this.
• Add a new "vcs_provider" to the API — but defaults to "github" for compatibility.

How did you test this code?

• Unit tests.
• Performance tests in staging.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
docs	Ready	Preview, Comment	May 8, 2026 10:57pm

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-preview	Ignored		May 8, 2026 10:57pm
flagsmith-frontend-staging	Ignored		May 8, 2026 10:57pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.44%. Comparing base (e4651d1) to head (b809ab4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7463   +/-   ##
=======================================
  Coverage   98.44%   98.44%           
=======================================
  Files        1398     1399    +1     
  Lines       52654    52705   +51     
=======================================
+ Hits        51834    51885   +51     
  Misses        820      820           

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