multi-stage Dockerfile + player recap styles

Dockerfile: split into whisperx-base and runtime stages so app-only
changes skip the 10min pip install on rebuild. Example compose updated.

Player recap: replace hardcoded 'story so far' prompt with selectable
styles (story/diary/bullets/custom) via dropdown in Setup and Settings.

Settings: add NAT20_ONBOARDING_COMPLETED env var; fix merge logic so
env vars properly override defaults (not just empty DB values).

docker-compose.example.yml

@@ -1,9 +1,24 @@
 # Example Docker Compose — uses a named volume so data persists across rebuilds
 # without needing a specific host path. Copy to docker-compose.yml and customise.
 services:
+  whisperx-base:
+    build:
+      context: ./backend
+      target: whisperx-base
+    image: nat20-whisperx-base:latest
+    restart: "no"
+
   backend:
-    build: ./backend
+    build:
+      context: ./backend
+      target: runtime
+    image: nat20-backend:latest
     restart: unless-stopped
+    depends_on:
+      - whisperx-base
+    environment:
+      # Skip setup wizard on container restart (or "false" to force re-run)
+      NAT20_ONBOARDING_COMPLETED: "true"
     volumes:
       - app_data:/data
       - hf_cache:/root/.cache/huggingface