perf: speed up UserAdmin change page inline

[thomasrockhu-codecov]

Summary

The Django admin User change page (codecov_auth/user/{id}/change/) is the #3 most expensive admin page in Sentry (avg ~7s, p95 ~13.7s) — but its DB time is trivial (<1s). The cost is rendering: AccountsUsersInline had no autocomplete, so every inline form rendered a <select> of every Account, loading the whole table (a cursor over all rows) and calling Account.__str__ per option.

This PR:

• Adds autocomplete_fields = ("account",) to AccountsUsersInline so account choices load on demand (mirrors AccountsUsersAdmin, InvoiceBillingAdmin, StripeBillingAdmin).
• Sets extra = 0 so no blank widget-bearing form is built on every view.
• Adds get_queryset(...).select_related("account", "user") to avoid an N+1 when rendering existing rows.

No schema/migration change.

Measured impact

Local profile with 2,000 accounts:

	Before	After
Full-table Account scans	6	0
Wall time	~607 ms	~100 ms
Queries	12	10

Remaining account queries are bounded WHERE id= label lookups for selected rows.

Audit (same anti-pattern elsewhere)

Checked every other codecov_auth change page: OwnerAdmin (bot/account already autocomplete, user readonly), InvoiceBillingInline/StripeBillingInline (only FK is the parent link), OwnerUserInline/OwnerOrgInline/Account.users (readonly), AccountAdmin/SentryUserAdmin/OwnerExportAdmin (readonly/no big FK). Only AccountsUsersInline was affected.

Test plan

• Existing codecov_auth/tests/test_admin.py User/Account/inline tests pass (19 passed)
• Local profile confirms 0 full-table Account scans post-fix
• Verify in staging admin that the account field is an autocomplete and still saves correctly
• Watch Sentry codecov_auth/user/{id}/change/ p95 after deploy

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (701e991) to head (2e72681).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1418   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files        1325     1325           
  Lines       50868    50871    +3     
  Branches     1626     1626           
=======================================
+ Hits        46744    46747    +3     
  Misses       3818     3818           
  Partials      306      306           

Flag	Coverage Δ
apiunit	94.94% <100.00%> (+<0.01%)	⬆️

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[codecov-notifications]

Codecov Report

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

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