feat: Integrate ASP.NET Core Identity for user authentication

This commit integrates ASP.NET Core Identity into the application to enable user registration, login, and management. This lays the groundwork for securing data per user.

**Key Changes:**

*   **DbContext Configuration:**
    *   Modified `ApplicationDbContext.cs` to inherit from `IdentityDbContext<IdentityUser>`.
    *   Removed an unnecessary `using` statement from `ApplicationDbContext.cs`.

*   **Program.cs Setup:**
    *   Configured `AddDefaultIdentity<IdentityUser>` with `AddEntityFrameworkStores<ApplicationDbContext>()` to register Identity services.
    *   Ensured correct ordering of `UseAuthentication()` and `UseAuthorization()` middleware.
    *   Added `app.MapRazorPages()` to enable the Identity UI pages.
    *   Verified core package versions in `turf_tasker.csproj` for consistency across EF Core and Identity components (`8.0.6`).

*   **Identity UI:**
    *   Scaffolded ASP.NET Core Identity pages (Login, Register, Manage, etc.) to provide the user interface for authentication.
    *   Added a `_LoginPartial.cshtml` partial view to the `Views/Shared` folder.
    *   Rendered `_LoginPartial` in `Views/Shared/_Layout.cshtml` to display login/register/logout links in the navigation bar.

*   **Migrations:**
    *   Created and applied a new migration (`AddIdentitySchema`) to create the necessary ASP.NET Core Identity database tables (e.g., `AspNetUsers`, `AspNetRoles`).

Areas/Identity/Pages/Account/LoginWith2fa.cshtml

@@ -0,0 +1,39 @@
+@page
+@model LoginWith2faModel
+@{
+    ViewData["Title"] = "Two-factor authentication";
+}
+
+<h1>@ViewData["Title"]</h1>
+<hr />
+<p>Your login is protected with an authenticator app. Enter your authenticator code below.</p>
+<div class="row">
+    <div class="col-md-4">
+        <form method="post" asp-route-returnUrl="@Model.ReturnUrl">
+            <input asp-for="RememberMe" type="hidden" />
+            <div asp-validation-summary="ModelOnly" class="text-danger" role="alert"></div>
+            <div class="form-floating mb-3">
+                <input asp-for="Input.TwoFactorCode" class="form-control" autocomplete="off" />
+                <label asp-for="Input.TwoFactorCode" class="form-label"></label>
+                <span asp-validation-for="Input.TwoFactorCode" class="text-danger"></span>
+            </div>
+            <div class="checkbox mb-3">
+                <label asp-for="Input.RememberMachine" class="form-label">
+                    <input asp-for="Input.RememberMachine" />
+                    @Html.DisplayNameFor(m => m.Input.RememberMachine)
+                </label>
+            </div>
+            <div>
+                <button type="submit" class="w-100 btn btn-lg btn-primary">Log in</button>
+            </div>
+        </form>
+    </div>
+</div>
+<p>
+    Don't have access to your authenticator device? You can
+    <a id="recovery-code-login" asp-page="./LoginWithRecoveryCode" asp-route-returnUrl="@Model.ReturnUrl">log in with a recovery code</a>.
+</p>
+
+@section Scripts {
+    <partial name="_ValidationScriptsPartial" />
+}