Phase 8: optional accounts, per-user data, shared demo key

Guest-first multi-user mode behind AIDND_MULTI_USER (local installs
unchanged): signed-cookie guest sessions bootstrapped by /api/auth/me,
register upgrades the guest in place, login/logout, per-IP rate limits.
Every router scoped by user_id; Settings become per-user with the API
key Fernet-encrypted at rest and write-only through the API. Users
without a key get a server-funded demo key (OpenRouter free models,
20 turns/day, memory bank disabled on demo turns). Public read-only
demo scenarios (seed_demo.py); debug log restricted to local mode.
Frontend: auth modal + guest nudge, 401 re-establish/retry, demo
banner and key management in Settings.

Migrations 13-23 adopt existing data under a local user and encrypt
stored keys. Verified: migration on a copy of real data.db, two-session
isolation + register/login via curl and Chrome, demo cap 429, live
OpenRouter turn through the encrypted-key path, vite build + oxlint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KFsGHju9szibJJa2YJcdbg

backend/app/schemas.py

@@ -66,6 +66,7 @@ class ScenarioUpdate(BaseModel):
 
 class ScenarioOut(ORMModel, ScenarioBase):
     id: int
+    is_public: bool = False  # shared demo content — read-only for everyone
     created_at: datetime
     updated_at: datetime
     story_cards: list[StoryCardOut] = []
@@ -77,6 +78,7 @@ class ScenarioListItem(ORMModel):
     title: str
     description: str
     tags: str
+    is_public: bool = False
     updated_at: datetime
 
 
@@ -226,11 +228,19 @@ class AdventureScriptUpdate(BaseModel):
     output_js: str | None = None
 
 
+# ---------- Auth (Phase 8) ----------
+
+class AuthCredentials(BaseModel):
+    email: str
+    password: str
+
+
 # ---------- Settings ----------
 
 class SettingsOut(ORMModel):
     endpoint_url: str
-    api_key: str
+    # The key itself is never echoed back (encrypted at rest, write-only).
+    has_api_key: bool
     model: str
     api_mode: str
     temperature: float