Fix DbContext concurrency error in parallel company verification

Use IDbContextFactory pattern to create isolated DbContext instances
for each cache operation, making parallel verification thread-safe.

Changes:
- Add IDbContextFactory<ApplicationDbContext> registration
- Update CompanyVerifierService to use factory for cache operations
- Update tests with InMemoryDatabaseRoot for shared test data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/TrueCV.Infrastructure/Services/CompanyVerifierService.cs

@@ -14,7 +14,7 @@ namespace TrueCV.Infrastructure.Services;
 public sealed class CompanyVerifierService : ICompanyVerifierService
 {
     private readonly CompaniesHouseClient _companiesHouseClient;
-    private readonly ApplicationDbContext _dbContext;
+    private readonly IDbContextFactory<ApplicationDbContext> _dbContextFactory;
     private readonly ILogger<CompanyVerifierService> _logger;
 
     private const int FuzzyMatchThreshold = 70;
@@ -22,11 +22,11 @@ public sealed class CompanyVerifierService : ICompanyVerifierService
 
     public CompanyVerifierService(
         CompaniesHouseClient companiesHouseClient,
-        ApplicationDbContext dbContext,
+        IDbContextFactory<ApplicationDbContext> dbContextFactory,
         ILogger<CompanyVerifierService> logger)
     {
         _companiesHouseClient = companiesHouseClient;
-        _dbContext = dbContext;
+        _dbContextFactory = dbContextFactory;
         _logger = logger;
     }
 
@@ -123,8 +123,11 @@ public sealed class CompanyVerifierService : ICompanyVerifierService
     {
         var cutoffDate = DateTime.UtcNow.AddDays(-CacheExpirationDays);
 
+        // Use factory to create a new DbContext for thread-safe parallel access
+        await using var dbContext = await _dbContextFactory.CreateDbContextAsync();
+
         // Get recent cached companies
-        var cachedCompanies = await _dbContext.CompanyCache
+        var cachedCompanies = await dbContext.CompanyCache
             .Where(c => c.CachedAt >= cutoffDate)
             .ToListAsync();
 
@@ -160,7 +163,10 @@ public sealed class CompanyVerifierService : ICompanyVerifierService
 
     private async Task CacheCompanyAsync(CompaniesHouseSearchItem item)
     {
-        var existingCache = await _dbContext.CompanyCache
+        // Use factory to create a new DbContext for thread-safe parallel access
+        await using var dbContext = await _dbContextFactory.CreateDbContextAsync();
+
+        var existingCache = await dbContext.CompanyCache
             .FirstOrDefaultAsync(c => c.CompanyNumber == item.CompanyNumber);
 
         if (existingCache is not null)
@@ -183,10 +189,10 @@ public sealed class CompanyVerifierService : ICompanyVerifierService
                 CachedAt = DateTime.UtcNow
             };
 
-            _dbContext.CompanyCache.Add(cacheEntry);
+            dbContext.CompanyCache.Add(cacheEntry);
         }
 
-        await _dbContext.SaveChangesAsync();
+        await dbContext.SaveChangesAsync();
     }
 
     private static CompanyVerificationResult CreateVerificationResult(