overhaul: single-session deployment + redesigned frontend

Backend simplification:
- The server now loads ONE pool JSON from $QUIZ_POOL_PATH at startup and
  upserts a single canonical session. The session id comes from the pool
  JSON's optional "session_id" field, falling back to $QUIZ_SESSION_ID.
- The multi-quiz / multi-session CRUD API is gone:
    DELETED  GET/POST /admin/api/quizzes
    DELETED  POST    /admin/api/quizzes/upload
    DELETED  GET/POST /admin/api/sessions
    DELETED  GET     /admin/login (HTML stub)
    DELETED  GET     /admin/api/sessions/{sid}/csv (replaced by /admin/api/csv)
  Replaced with a single-session control surface:
    GET  /admin/                — serves admin.html unconditionally
    GET  /admin/api/state       — admin-gated; pool meta + state + QR + join URL
    POST /admin/api/reset       — admin-gated; wipe submissions + back to lobby
    POST /admin/logout          — clear admin cookie
    GET  /admin/api/csv         — single-session results
    WS   /ws/instructor/{sid}   — kept; new commands "next" + "reset"
- Instructor "Next" button is now a single state-driving command
  (RoomManager.advance_to_next): from lobby it opens Q0; from question_open
  it closes the current Q and opens the next; from question_closed it
  opens the next; if past the last question it ends the session.
- New RoomManager.reset wipes submissions, participants, and per-question
  state, then broadcasts a clean lobby.
- Student GET / now redirects to /?sid=<canonical> when no sid is given,
  so the QR / share URL is fully deterministic.

Frontend rewrite (functional baseline; visual polish to follow):
- /admin/ is now a single SPA: GET /admin/api/state decides login form
  vs dashboard. No separate /admin/login URL bar.
- Admin dashboard is state-driven with one primary action per state.
  QR code, join URL, and live participant list are always visible on the
  left so the operator can leave the page on a projector.
- Student answer buttons are big and tappable; reveal screen highlights
  correct/wrong choice + shows score, total, and rank.
- Static admin/student SPAs share a CSS palette with light/dark support.

Tests rewritten around the single canonical session id.
The auto-bootstrapped session lets each test fixture skip the old
quiz/session creation dance. 39/39 tests pass.

Cleanup:
- Deleted CODEX_PROMPT.md, IMPLEMENTATION_REPORT.md, NOTES.md, SPEC.md,
  static/observer.html (obsolete codex-build artifacts and the unused
  observer view).
- .gitignore now blocks /pool.json (the runtime file the operator drops
  on the server) and the leftover .codex_done / codex_run.log / etc.
- bootstrap.sh seeds /opt/quiz/pool.json from examples/pool_example.json
  on first deploy so a fresh box reaches a usable state without manual
  intervention; .env now includes QUIZ_POOL_PATH.

app/main.py

@@ -1,27 +1,47 @@
 from __future__ import annotations
 
+import logging
+from contextlib import asynccontextmanager
+
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
 
 from app import __version__
 from app.config import Settings
 from app.db import init_db
+from app.pool import PoolValidationError, load_pool_from_file
 from app.room import RoomManager
 from app.routes_admin import router as admin_router
 from app.routes_student import router as student_router
 
+log = logging.getLogger("quiz")
+
 
 def create_app(settings: Settings | None = None) -> FastAPI:
     settings = settings or Settings.from_env()
     rooms = RoomManager(settings)
-    app = FastAPI(title="Live In-Lecture Quiz Portal")
+
+    @asynccontextmanager
+    async def lifespan(_app: FastAPI):
+        await init_db(settings.db_path)
+        try:
+            pool = load_pool_from_file(settings.pool_path)
+        except PoolValidationError as exc:
+            log.error("Pool load failed at %s: %s", settings.pool_path, exc)
+            log.error("Server is starting without an active session.")
+            log.error("Drop a valid pool JSON at %s and restart.", settings.pool_path)
+        else:
+            sid = pool.get("session_id", settings.default_session_id)
+            await rooms.ensure_single_session(sid, pool)
+            rooms.canonical_sid = sid
+            log.info("Session ready: sid=%s title=%r questions=%d",
+                     sid, pool["title"], len(pool["questions"]))
+        yield
+
+    app = FastAPI(title="Live In-Lecture Quiz Portal", lifespan=lifespan)
     app.state.settings = settings
     app.state.rooms = rooms
 
-    @app.on_event("startup")
-    async def startup() -> None:
-        await init_db(settings.db_path)
-
     @app.get("/healthz")
     async def healthz():
         return {