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).

backend/Dockerfile

@@ -1,6 +1,7 @@
 # ==========================================
-# STAGE 1: Builder Environment
+# STAGE 1 — builder: compile Python deps
 # ==========================================
+# Changes only when requirements.txt is modified.
 FROM python:3.11-slim AS builder
 
 WORKDIR /build
@@ -28,25 +29,32 @@ RUN find /root/.local -type d -name "__pycache__" -exec rm -rf {} + \
     && find /root/.local -name "*.dist-info" -exec sh -c 'rm -f "$1"/RECORD "$1"/INSTALLER' _ {} \;
 
 # ==========================================
-# STAGE 2: Lightweight Final Runtime
+# STAGE 2 — whisperx-base: deps only, no app
 # ==========================================
-FROM python:3.11-slim AS runtime
+# Targeted by docker-compose for dev caching.
+# If you only need this layer: docker build --target=whisperx-base -t nat20-whisperx-base .
+FROM python:3.11-slim AS whisperx-base
 
-WORKDIR /app
-
-# ffmpeg is mandatory for the audio-extraction step.
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
-# Copy the (now stripped) site-packages from the builder stage. None of the
-# build-essential/gcc toolchain from Stage 1 ends up here.
 COPY --from=builder /root/.local /root/.local
-COPY app /app/app
 
 ENV PATH=/root/.local/bin:$PATH
 ENV PYTHONUNBUFFERED=1
 
+# ==========================================
+# STAGE 3 — runtime: full release image
+# ==========================================
+# Build for production: docker build --target=runtime -t nat20-notes-backend .
+# Layers app code on top of whisperx-base in a single Dockerfile pass.
+FROM whisperx-base AS runtime
+
+WORKDIR /app
+
+COPY app /app/app
+
 EXPOSE 8000
 
 CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]