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/config.py

@@ -46,4 +46,8 @@ DEFAULT_SETTINGS = {
     # Optional world context injected into every summarization prompt
     "world_context": "",
     "world_context_path": "",
+
+    # Player recap style: "story" | "diary" | "bullets" | "custom"
+    "player_recap_style": "story",
+    "player_recap_custom_prompt": "",
 }