perf: make CommitAdmin SHA search index-friendly

[thomasrockhu-codecov]

Summary

Searching the admin commits changelist by full SHA (e.g. ?q=cdd4a06d…) was taking a very long time against the ~172M row commits table.

The filter used commitid__iexact, which Django compiles to UPPER(commitid) = UPPER(...). Wrapping the column in a function means no btree index can serve the predicate, so Postgres fell back to a sequential scan — even though the same lookup written as WHERE commitid = '…' in psql is fast.

Changes to CommitAdmin.get_search_results:

• Match SHAs with exact lowercase equality (commitid=term.lower()) instead of __iexact. SHAs are hex and stored lowercase elsewhere in the app (GraphQL commit search already does search.lower()), so case-insensitive matching bought nothing and cost the index.
• Accept a combined <repoid> <full SHA> term, which filters on both columns and lets the query use the existing commits_repoid_commitid unique index. This is the fastest path and is now called out in search_help_text.
• Bare repoid and bare SHA searches still work as before.

No schema change. SHA-only search is now planned like the equivalent raw query; a standalone commitid index would be a separate follow-up if SHA-only lookups need to be fast without a repoid.

Test plan

• ruff format / ruff check clean on both changed files
• pytest core/tests/test_admin.py -k CommitAdmin (needs the docker Postgres; couldn't reach postgres host locally)
• In admin, search <repoid> <full SHA> and confirm the result loads quickly
• In admin, search a bare full SHA and confirm it returns the same commit
• In admin, search a bare repoid and confirm the changelist is unchanged

Made with Cursor

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (68c86d1) to head (bf63ec2).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1967   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files        1336     1336           
  Lines       53174    53177    +3     
  Branches     1647     1647           
=======================================
+ Hits        48715    48718    +3     
  Misses       4138     4138           
  Partials      321      321           

Flag	Coverage Δ
apiunit	93.94% <100.00%> (+<0.01%)	⬆️

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