Rename demo master-switch env var: DEMO_MODE → DEMO

[Ho1yShif]

Why

The demo master switch (the flag that turns on per-IP rate limiting + per-visitor ephemeral chat history for a public demo) is now spelled DEMO instead of DEMO_MODE, adopting DEMO as the convention.

Context: the public demo's Render env var was set as DEMO, but the code read DEMO_MODE, so demo mode silently stayed off (shared, disk-persisted history across visitors). Rather than keep the longer name, we standardize on DEMO.

What changed

• deeptutor/services/demo/rate_limiter.py — limiter now reads os.environ["DEMO"] (the one behavior change).
• render.yaml, README.md, .env.example — documented env var name.
• Docstrings/comments in main.py, routers/auth.py, routers/unified_ws.py, routers/chat.py, and web/lib/demo-visitor.ts.
• Demo test suite updated to set DEMO.

Left unchanged on purpose: DEMO_RATE_LIMIT_PER_MIN / DEMO_RATE_LIMIT_PER_HOUR (separate, descriptive tuning knobs) and the is_demo_mode() function name.

⚠️ Breaking change

Deployments that set DEMO_MODE must switch the env var to DEMO. The flag is off by default, so local/private forks are unaffected. The public demo service (srv-d9a693…) already has DEMO set, so this rename makes the running code match that key.

Verification

DEMO=true       -> is_demo_mode() == True
DEMO unset      -> is_demo_mode() == False
DEMO_MODE=true  -> is_demo_mode() == False   # old key no longer honored (expected)

• Demo test suite: pytest tests/services/demo tests/api/test_demo_visitor_isolation.py → 42 passed.
• pre-commit on all changed files: ruff, ruff-format, prettier, mypy, bandit, check-yaml, detect-secrets → all pass.

After merge

Deploy, then confirm isolation is live (fresh visitor ids must return an empty session list):

BASE=https://deeptutor-t1z4.onrender.com
curl -s -H "X-Demo-Visitor: probe-$(date +%s)a" "$BASE/api/v1/sessions?limit=5"
curl -s -H "X-Demo-Visitor: probe-$(date +%s)b" "$BASE/api/v1/sessions?limit=5"

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