CodeQL 16: refactor: remove useless upcasts via record types and default(T)

[rlorenzo]

Summary

Closes 6 of the 7 non-test cs/useless-upcast alerts by either extracting a named record or replacing (T?)null with default(T?):

• PhotoExportService.BatchLoadPhotosAsync (PhotoExportService.cs:906, 911, 916, 921) - replaces the 4 anonymous types with a private PhotoLookupResult(string MailId, byte[]? PhotoBytes) record. The catch branches now return new PhotoLookupResult(s.MailId, null) - no cast needed because the parameter is typed.
• VerificationService.cs:235 - (DateTime?)null → default(DateTime?). Same value, no cast.
• CliniciansController.cs:204 - (string?)null → default(string). The two anonymous types in the if/else assign to an object variable, so they don't need to match.
• _EmailLayout.cshtml:46,48 - extracted var baseUrl = ViewBag.BaseUrl as string; into a Razor block; the (string?)/(string) casts on every reference go away.

Not addressed

The 1 remaining non-test useless-upcast alert is essentially the same site as above - they collapse together once CodeQL re-scans.

Context

Sixteenth in the CodeQL N: cleanup series.

Test plan

• npm run test:backend - 1946 tests passing
• npm run verify:build - clean
• Pre-commit lint+test+verify all passed

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• Commit unit tests in branch codeql/16-extract-record-types

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[codecov-commenter]

Bundle Report

Bundle size has no change ✅

[codecov-commenter]

Codecov Report

❌ Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.96%. Comparing base (38de1ad) to head (e1282ea).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
web/Areas/Students/Services/PhotoExportService.cs	0.00%	5 Missing ⚠️
...nicalScheduler/Controllers/CliniciansController.cs	0.00%	1 Missing ⚠️
...eb/EmailTemplates/Views/Shared/_EmailLayout.cshtml	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #202   +/-   ##
=======================================
  Coverage   42.96%   42.96%           
=======================================
  Files         877      877           
  Lines       51468    51470    +2     
  Branches     4802     4802           
=======================================
+ Hits        22113    22114    +1     
- Misses      28831    28832    +1     
  Partials      524      524           

Flag	Coverage Δ
backend	43.04% <22.22%> (+<0.01%)	⬆️
frontend	41.34% <ø> (ø)

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