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/app/routers/settings.py

@@ -13,6 +13,7 @@ router = APIRouter(prefix="/api/settings", tags=["settings"])
 
 # Env var overrides — set these in docker-compose.yml to prefill the setup wizard
 _ENV_OVERRIDES = {
+    "onboarding_completed": "NAT20_ONBOARDING_COMPLETED",
     "hf_token": "NAT20_HF_TOKEN",
     "whisper_model": "NAT20_WHISPER_MODEL",
     "whisper_compute_type": "NAT20_WHISPER_COMPUTE_TYPE",
@@ -36,13 +37,16 @@ def _coerce(key: str, value: str):
 
 
 def _merge_with_env(raw: dict) -> dict:
-    """DB values win, env vars fill gaps, DEFAULT_SETTINGS fill remaining gaps."""
-    merged = {**config.DEFAULT_SETTINGS, **raw}
+    """Priority: explicit user DB saves (non-default) > env vars > DEFAULT_SETTINGS."""
+    merged = {**config.DEFAULT_SETTINGS}
     for key, env_name in _ENV_OVERRIDES.items():
-        if not raw.get(key, "").strip():
-            val = os.environ.get(env_name)
-            if val is not None:
-                merged[key] = val
+        val = os.environ.get(env_name)
+        if val is not None:
+            merged[key] = val
+    for key, val in raw.items():
+        db_val = val.strip()
+        if db_val and db_val != config.DEFAULT_SETTINGS.get(key, ""):
+            merged[key] = db_val
     return merged