feat: add Docker Hub image tags and update CI workflows

- docker-compose.yml: add image: kalekber/libnovel-{backend,runner,ui,caddy}:${GIT_TAG} alongside each build: block — prod pulls from Docker Hub, dev builds locally with just build
- justfile: add push, build-push, pull-images, pull-infra recipes
- ci-v3.yaml: fix v3/ path references, add caddy job, add registry layer cache
- release-v3.yaml: fix v3/ path references, add caddy job, simplify tag pattern to semver (v*), add layer cache, caddy added to release gate

docker-compose.yml

@@ -128,6 +128,7 @@ services:
 
   # ─── Backend API ──────────────────────────────────────────────────────────────
   backend:
+    image: kalekber/libnovel-backend:${GIT_TAG:-latest}
     build:
       context: ./backend
       dockerfile: Dockerfile
@@ -167,6 +168,7 @@ services:
 
   # ─── Runner (background task worker) ─────────────────────────────────────────
   runner:
+    image: kalekber/libnovel-runner:${GIT_TAG:-latest}
     build:
       context: ./backend
       dockerfile: Dockerfile
@@ -219,6 +221,7 @@ services:
 
   # ─── SvelteKit UI ─────────────────────────────────────────────────────────────
   ui:
+    image: kalekber/libnovel-ui:${GIT_TAG:-latest}
     build:
       context: ./ui
       dockerfile: Dockerfile
@@ -318,6 +321,7 @@ services:
   # Custom build includes github.com/mholt/caddy-ratelimit and
   # github.com/hslatman/caddy-crowdsec-bouncer/http.
   caddy:
+    image: kalekber/libnovel-caddy:${GIT_TAG:-latest}
     build:
       context: ./caddy
       dockerfile: Dockerfile