Phase 9: production hardening

Config via env, abuse/resource limits, and production serving so the app
is safe to expose publicly:

- Fail-fast on missing SECRET_KEY when MULTI_USER=true
- quickjs per-execution time/memory limits (while(true) can't hang server)
- Per-user/per-IP rate limiting on turn/script/auth endpoints
- Request body size limit + per-user row caps
- Security headers (CSP, X-Frame-Options, nosniff, referrer-policy) incl. SSE
- Debug router 403 and /docs disabled in multi-user mode
- DATABASE_URL support (defaults to Neon Postgres) alongside SQLite
- Documented all env vars in backend/.env.example

Verified locally via uvicorn (MULTI_USER=1, SQLite); see plan/09-phase-hardening.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017e6tQuojBLYPetUfmhit4X

backend/app/routers/story_cards.py

@@ -1,7 +1,7 @@
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.orm import Session
 
-from .. import auth, models, schemas
+from .. import auth, limits, models, schemas
 from ..database import get_db
 
 router = APIRouter(prefix="/api/story-cards", tags=["story-cards"])
@@ -50,6 +50,10 @@ def create_story_card(
     owner = db.get(owner_model, owner_id)
     if owner is None or owner.user_id != user.id:
         raise HTTPException(404, "Owner not found")
+    limits.check_row_cap(
+        "story_cards", db, user,
+        scenario_id=payload.scenario_id, adventure_id=payload.adventure_id,
+    )
     card = models.StoryCard(**payload.model_dump())
     db.add(card)
     db.commit()