feat: migrate imphnen-backend-qr into workspace as imphnen-qr crate

Ports the Go QR campaign overlay service to a self-contained Rust crate
nested at /v1/qr/... in the gateway.

Features:
- Auth: register, login, Google OAuth, JWT refresh (bcrypt compat with Go DB)
- Users: profile management + admin CRUD (list/role/delete)
- Campaigns: create (auto-generates QR PNG via qrcode crate), list,
  activate, delete; process-image endpoint overlays active campaign QR
  onto uploaded images (bottom-right corner, image crate)
- QR pool connects to imphnen_qr database via QR_DATABASE_URL

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

imphnen-qr/src/users/application/mod.rs

@@ -0,0 +1 @@
+pub mod user_service;

imphnen-qr/src/users/application/user_service.rs

@@ -0,0 +1,51 @@
+use async_trait::async_trait;
+use imphnen_utils::errors::AppError;
+use std::sync::Arc;
+use uuid::Uuid;
+
+use crate::users::domain::{
+    entity::{UpdateUserInput, UserEntity},
+    repository::UserRepository,
+    service::QrUserService,
+};
+
+pub struct QrUserServiceImpl {
+    repo: Arc<dyn UserRepository>,
+}
+
+impl QrUserServiceImpl {
+    pub fn new(repo: Arc<dyn UserRepository>) -> Self {
+        Self { repo }
+    }
+}
+
+#[async_trait]
+impl QrUserService for QrUserServiceImpl {
+    async fn get_profile(&self, user_id: Uuid) -> Result<UserEntity, AppError> {
+        self.repo
+            .find_by_id(user_id)
+            .await?
+            .ok_or_else(|| AppError::NotFoundError("User not found".to_string()))
+    }
+
+    async fn update_profile(&self, user_id: Uuid, input: UpdateUserInput) -> Result<UserEntity, AppError> {
+        if let Some(ref email) = input.email {
+            if email.trim().is_empty() {
+                return Err(AppError::ValidationError("Email cannot be empty".to_string()));
+            }
+        }
+        self.repo.update(user_id, input).await
+    }
+
+    async fn list_all(&self) -> Result<Vec<UserEntity>, AppError> {
+        self.repo.find_all().await
+    }
+
+    async fn update_role(&self, id: Uuid, role: String) -> Result<UserEntity, AppError> {
+        self.repo.update_role(id, role).await
+    }
+
+    async fn delete(&self, id: Uuid) -> Result<(), AppError> {
+        self.repo.delete(id).await
+    }
+}