Refactor nunit tests off InMemory/WebApplicationFactory, remove tuple returns (#4)

## Summary
- Extract `IMicCheckDbContext` and mock DB access with Moq instead of the EF InMemory provider, per CLAUDE.md testing guidelines.
- Rewrite HTTP-pipeline tests to exercise controllers/auth handlers directly instead of `WebApplicationFactory`.
- Remove tuple return types across the API in favor of named records (`PagedResult<T>`, `ProfileUpdateResult`, `ApiKeyCreationResult`).

## Test plan
- [x] `dotnet build` (full solution)
- [x] `dotnet test tests/api/MicCheck.Api.Tests.Unit` — 187/187 pass
- [x] `dotnet test tests/api/MicCheck.Api.Tests.Integration` — 8/8 pass (live API + Postgres via Aspire)
- [x] Verified Aspire AppHost/dashboard starts and API responds

Reviewed-on: #4

src/api/MicCheck.Api/Segments/SegmentService.cs

@@ -9,7 +9,7 @@ public record SegmentConditionDefinition(string Property, SegmentConditionOperat
 
 public record SegmentRuleDefinition(SegmentRuleType Type, IReadOnlyList<SegmentConditionDefinition> Conditions, IReadOnlyList<SegmentRuleDefinition>? ChildRules = null);
 
-public class SegmentService(MicCheckDbContext db, AuditService auditService)
+public class SegmentService(IMicCheckDbContext db, AuditService auditService)
 {
     private const int MaxSegmentsPerProject = 100;
     private const int MaxConditionsPerSegment = 100;