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/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "imphnen-qr"
+version = "0.2.0"
+edition = "2024"
+
+[dependencies]
+imphnen-utils.workspace = true
+axum.workspace = true
+axum-extra.workspace = true
+async-trait.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tokio.workspace = true
+jsonwebtoken.workspace = true
+bcrypt.workspace = true
+chrono.workspace = true
+uuid.workspace = true
+sqlx.workspace = true
+reqwest.workspace = true
+oauth2.workspace = true
+tracing.workspace = true
+utoipa.workspace = true
+image.workspace = true
+qrcode.workspace = true