refactor: move buildable .NET source into src/, update CI/doc paths

Groups the repo root into src (buildable source), docs (already existed),
and everything else (db/, scripts/, tests/, deploy/ - deployment/ops/test
assets that aren't compiled, already organized as their own folders). CI
now only needs src/ to build: dotnet restore/build/test/publish all point
at src/TaxBaik.sln, src/TaxBaik.Web/, src/TaxBaik.Proxy/.

- git mv every project (Domain, Infrastructure, Application,
  Application.Tests, Web, Web.Client, Proxy) and TaxBaik.sln into src/ as a
  unit, so relative ProjectReference/.sln paths stay valid unchanged.
- .gitea/workflows/deploy.yml: 6 dotnet restore/clean/build/test/publish
  invocations now point at src/. db/migrations and scripts/ stay at root
  (deploy_gb.sh and browser-e2e.yml only touch published output and the
  deployed URL, not source paths - verified, no changes needed there).
- scripts/validate_admin_render.sh: admin render-mode file paths now
  src/TaxBaik.Web.Client/...
- scripts/validate_kst_timestamps.sh: dropped deploy.sh from its target
  list - that script was removed in the prior cleanup commit (dead, no
  CI workflow referenced it) but this validator still expected it to exist.
- CLAUDE.md, docs/ENGINEERING_HARNESS.md, docs/ADMIN_PATTERN_CRITIQUE_WBS.md:
  updated project-structure diagram, dotnet run/build commands, and grep
  targets to the new src/ paths (also fixed a pre-existing stale path in
  ADMIN_PATTERN_CRITIQUE_WBS.md that still said TaxBaik.Web/Components/Admin
  from before that ever moved to TaxBaik.Web.Client).
- Added a Repo Root harness rule + Architecture Guardrail entries: new files
  belong under src/docs/tests/scripts/db/deploy, not loose at root; temp
  work stays outside the repo (or under a gitignored .scratch/) and is
  never committed.

Verified locally: dotnet build/test src/TaxBaik.sln (26/26 tests), and all
three scripts/validate_*.sh pass against the new layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/TaxBaik.Web/Pages/Portal/Login.cshtml.cs

@@ -0,0 +1,56 @@
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using TaxBaik.Web.Services;
+
+namespace TaxBaik.Web.Pages.Portal;
+
+public class LoginModel : PageModel
+{
+    private readonly PortalAuthService _portalAuthService;
+
+    [BindProperty]
+    public string Email { get; set; } = "";
+
+    [BindProperty]
+    public string Password { get; set; } = "";
+
+    [BindProperty]
+    public string? ErrorMessage { get; set; }
+
+    public LoginModel(PortalAuthService portalAuthService)
+    {
+        _portalAuthService = portalAuthService;
+    }
+
+    public void OnGet()
+    {
+    }
+
+    public async Task<IActionResult> OnPostAsync()
+    {
+        if (string.IsNullOrWhiteSpace(Email) || string.IsNullOrWhiteSpace(Password))
+        {
+            ErrorMessage = "이메일과 비밀번호를 입력하세요.";
+            return Page();
+        }
+
+        var signedIn = await _portalAuthService.SignInAsync(Email, Password);
+        if (!signedIn)
+        {
+            ErrorMessage = "로그인 정보를 확인할 수 없습니다.";
+            return Page();
+        }
+
+        return RedirectToPage("/Portal/Index");
+    }
+
+    public IActionResult OnPostGoogle() => Challenge(BuildProps("google"), PortalOAuthDefaults.GoogleScheme);
+
+    public IActionResult OnPostNaver() => Challenge(BuildProps("naver"), PortalOAuthDefaults.NaverScheme);
+
+    public IActionResult OnPostKakao() => Challenge(BuildProps("kakao"), PortalOAuthDefaults.KakaoScheme);
+
+    private static AuthenticationProperties BuildProps(string provider) =>
+        new() { RedirectUri = $"/taxbaik/portal/external-callback?provider={provider}" };
+}