deploy: add bootstrap.sh + Caddyfile + systemd unit + demo pool

One-shot deploy for fresh Ubuntu 24.04 root SSH:
  curl -fsSL https://gitea.ahkhan.me/apps/quiz/raw/branch/master/deploy/bootstrap.sh | bash

bootstrap.sh: idempotent stage-by-stage installer for Caddy, Python venv,
quiz system user, repo clone to /opt/quiz, env-var prompts, systemd unit,
Caddyfile, and a healthz check. Reattaches /dev/tty so curl|bash can read
the admin password interactively.

quiz.service: uvicorn under the quiz system user (no shell, no SSH),
ProtectSystem=full, ProtectHome=true, PrivateTmp=true, NoNewPrivileges=true.

Caddyfile.tpl: reverse_proxy 127.0.0.1:8001 with auto Let's Encrypt;
DOMAIN substituted at install time.

examples/pool_example.json: generic demo pool, schema reference only.

README rewritten around the deploy flow + class-day lifecycle.

examples/pool_example.json

@@ -0,0 +1,31 @@
+{
+  "title": "Demo Pool: Generic Knowledge",
+  "score_fn": "linear_decay",
+  "time_limit_default": 60,
+  "questions": [
+    {
+      "id": "demo1",
+      "text": "Which of these is a programming language?",
+      "options": {
+        "A": "HTTP",
+        "B": "Python",
+        "C": "TCP",
+        "D": "DNS"
+      },
+      "correct": "B",
+      "explanation": "Python is a general-purpose programming language; the others are network protocols."
+    },
+    {
+      "id": "demo2",
+      "text": "What is 2 + 2?",
+      "options": {
+        "A": "3",
+        "B": "4",
+        "C": "5",
+        "D": "22"
+      },
+      "correct": "B",
+      "explanation": "Basic arithmetic."
+    }
+  ]
+}