Isolate demo chat history per visitor

[Ho1yShif]

Problem

Opening the public demo in a second browser (or incognito) showed another visitor's chat history. This is a session-isolation bug, distinct from the earlier ephemeral-history fix (#4).

Root cause: DeepTutor is single-user by design — the sessions table has no owner column, and GET /api/v1/sessions lists sessions with no filter. In DEMO_MODE the SQLite session store is a single process-wide in-memory instance shared by every visitor, and all anonymous visitors resolve to the same local_admin_user. So the history endpoint returned everyone's chats to everyone. The ephemeral-history PR stopped disk writes but did nothing to isolate concurrent visitors.

Fix

Carry a per-visitor id (minted client-side, stored in localStorage) and key the demo in-memory store by it, so each visitor gets a physically separate ephemeral DB.

• deeptutor/services/demo/visitor.py — request-local visitor-id ContextVar (mirrors multi_user.context) with input sanitization. This is isolation, not authentication: an opaque token that only partitions the demo's ephemeral store.
• get_sqlite_session_store() — demo branch keyed per visitor. Each in-memory store already mints a unique db_path, so get_turn_runtime_manager() (cached by db_path) stays isolated per visitor automatically — including the WS turn path.
• Binding at the two existing choke points that set the current user: a new async bind_demo_visitor dependency in the _auth list (HTTP, reads X-Demo-Visitor header) and ws_require_auth (WS, reads ?visitor= query param, since WS upgrades can't set custom headers). No-op unless DEMO_MODE.
• Frontend — getDemoVisitorId() persists a random id in localStorage; apiFetch attaches the header and the unified WS appends the query param. An incognito window has its own storage → distinct id → distinct history.
• Also fixes a latent mypy error in SQLiteSessionStore.__init__ surfaced by touching the file (db_path is str | Path across the in-memory / on-disk branches).

Tests

• tests/services/demo/test_demo_visitor_isolation.py — store-level isolation (A can't see B; same id reuses one store; non-demo ignores the id).
• tests/api/test_demo_visitor_isolation.py — end-to-end through the ASGI stack: the bind_demo_visitor dependency sets the ContextVar visibly to the endpoint (guards the #481 sync-dependency regression class), a no-header request does not inherit a prior visitor's bucket, and ws_require_auth binds the query-param id.

Full suite: 2433 passed; the only failures are pre-existing missing-optional-dependency gaps (telegram/slack/croniter) unrelated to this change. ruff / ruff-format / mypy / bandit green on changed files; frontend test:node (155) + eslint green.

Scope & trade-offs

• Isolates the SQLite-backed history — the reported leak: /api/v1/sessions, dashboard recent, and unified WS turns.
• Not covered: the legacy JSON-manager /chat path and per-visitor memory persistence (the modern UI uses /api/v1/ws + /api/v1/sessions).
• Per-visitor in-memory stores accumulate until restart; acceptable for a demo given per-IP rate limiting and deploy restarts. A TTL/LRU is a possible follow-up.

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