Feat: Implement JWT bearer token authentication

Protects image upload, delete, and tag-update endpoints behind
Bearer token auth. Public read endpoints remain open. Angular SPA
gains a login page, auth interceptor, and route guard for /upload.

- JWTAuthProvider (HS256, sub/iat/exp, secrets.compare_digest)
- POST /api/v1/auth/token login endpoint
- require_auth FastAPI dependency on all write routes
- AuthService, LoginComponent, authInterceptor, authGuard
- Detail page hides write controls for unauthenticated visitors
- 43 unit tests passing; integration tests require Docker stack

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/app/auth/auth.guard.ts

@@ -0,0 +1,12 @@
+import { inject } from '@angular/core';
+import { CanActivateFn, Router } from '@angular/router';
+import { AuthService } from './auth.service';
+
+export const authGuard: CanActivateFn = (_route, state) => {
+  const auth = inject(AuthService);
+  const router = inject(Router);
+  if (auth.isAuthenticated()) {
+    return true;
+  }
+  return router.createUrlTree(['/login'], { queryParams: { returnUrl: state.url } });
+};