feat(api,core,infra): refactor endpoint routing and add institution account discovery

- Move login and refresh endpoints under a unified /auth route group
- Relocate institution connection logic from /accounts to /institutions
- Implement GET /institutions/{id}/available to fetch real-time Plaid
accounts
- Introduce HttpRequestBuilder utility to streamline integration testing
- Enhance PlaidService with GetAccountsAsync to support account fetching
- Clean up global usings and project structure for better domain
isolation

src/FiscalOS.API/Institutions/GetAvailable/Endpoint.cs

@@ -0,0 +1,50 @@
+namespace FiscalOS.API.Institutions.GetAvailable;
+
+internal static class Endpoint
+{
+  private const string Route = "/{id}/available";
+
+  public static RouteHandlerBuilder MapGetAvailableEndpoint(this RouteGroupBuilder groupBuilder)
+  {
+    return groupBuilder.MapGet(Route, HandleAsync);
+  }
+
+  private static async Task<IResult> HandleAsync(
+    HttpContext httpContext,
+    [FromRoute] Guid id,
+    [FromServices] AppDbContext appDbContext,
+    [FromServices] PlaidService plaidService,
+    [FromServices] IEncryptor encryptor,
+    CancellationToken ct
+  )
+  {
+    var userId = httpContext.GetUserId();
+
+    var user = await appDbContext.Users
+      .Include(u => u.Institutions.Where(i => i.Id == id))
+      .ThenInclude(i => i.Metadata)
+      .SingleOrDefaultAsync(u => u.Id == userId, ct);
+
+    if (user is null)
+    {
+      return Results.Unauthorized();
+    }
+
+    if (user.Institutions.Any() is false)
+    {
+      return Results.NotFound();
+    }
+
+    var institution = user.Institutions.First();
+
+    if (institution.Metadata is not PlaidMetadata plaidMetadata)
+    {
+      return Results.BadRequest();
+    }
+
+    var accessToken = await encryptor.DecryptAsyncFor(user, plaidMetadata.EncryptedAccessToken, ct);
+    var accounts = await plaidService.GetAccountsAsync(accessToken);
+
+    return Results.Ok(accounts);
+  }
+}