feat: anti-cheat + presence panel + projector view

Refinement pass on top of the validated v1.2 base.

Hardening / quick fixes
- Caddyfile.tpl: CSP / HSTS / X-Frame / Referrer-Policy / Permissions-Policy,
  1 MiB request body cap, X-Forwarded-For pass-through; stock-Caddy compatible.
- auth.py: STUDENT_MAX_AGE 1y -> 30d.
- bootstrap.sh: stage 5 prepends `git config --add safe.directory` so the
  re-bootstrap path no longer 'detected dubious ownership' faults.
- admin.js: drop the misleading `closedPayload?.state || session.state`
  shim; state derives from session only.

Anti-cheat
- New `student_events` audit table; new POST /api/session/{sid}/event for
  blur / visibility_hidden / focus / visibility_visible. quiz.js debounces
  events at 1.5s and uses sendBeacon for visibility_hidden so the event
  survives a navigation. Counts surface in admin presence + CSV export.
- First-claim-wins on join: add_participant raises DuplicateStudentId on
  PK violation; route returns 409 + records a duplicate_join audit event
  with attempted name + IP + UA. Admin dashboard surfaces a per-row red
  badge for hits on real participants and a top-of-page alert for orphan
  attempts.
- DELETE /admin/api/students/{id} as the recovery hatch: clears the
  participant + submissions, kicks active WS sockets so a stale cookie
  cannot continue submitting. quiz.js surfaces the FastAPI detail message
  in the join form so users see the 'already in use' guidance.

Presence panel
- New presence_update WS message; in-process presence map keyed on
  student_id tracks ws_count + last_seen_ms. Admin dashboard renders
  per-student rows: connected/idle/dropped dot, blur+hidden+duplicate
  badges, 'answered current Q' tick, and a clear-student button.

Projector view (public, read-only)
- /projector/?sid=..., GET /api/session/{sid}/projector, WS
  /ws/projector/{sid}. Single self-contained projector_state snapshot
  pushed on every state change. Public leaderboard strips student_id;
  QR rendered server-side as data: URL (CSP-compliant).
- Includes per-Q answer histogram, 8-bucket response-time distribution,
  10-bucket score distribution.
- static/projector.{html,css,js}: editorial-broadside design — masthead,
  registration crosses, conic-gradient countdown ring, SVG stepped-area
  score distribution with median tick, leaderboard row-stagger. Inherits
  light/dark tokens from style.css; honours prefers-reduced-motion. No
  scroll at 1366x768 / 1920x1080 / 3440x1440.

Tests
- tests/test_anti_cheat.py: blur logging + CSV count, unknown-kind 422,
  unauthenticated event 401, duplicate-join 409 + audit, admin
  clear-student happy + 404, server-side submit lockout regression.
- tests/test_projector.py: snapshot shape, leaderboard student_id
  redaction, WS push on state change, 404 for unknown sid, page redirect
  when no sid.
- Existing tests updated for the new presence_update snapshot frame +
  CSV header columns + first-claim-wins refusal of re-key.

57/57 pytest green; smoke-tested locally end-to-end.

app/csv_export.py

@@ -11,7 +11,21 @@ from app.db import connect
 async def export_session_csv(db_path: str, sid: str) -> str:
     out = StringIO()
     writer = csv.writer(out)
-    writer.writerow(["sid", "student_id", "name", "question_idx", "answer", "elapsed_ms", "score", "status"])
+    writer.writerow(
+        [
+            "sid",
+            "student_id",
+            "name",
+            "question_idx",
+            "answer",
+            "elapsed_ms",
+            "score",
+            "status",
+            "blur_count",
+            "hidden_count",
+            "duplicate_join_attempts",
+        ]
+    )
     async with connect(db_path) as db:
         cursor = await db.execute(
             """
@@ -24,7 +38,21 @@ async def export_session_csv(db_path: str, sid: str) -> str:
             (sid,),
         )
         rows = await cursor.fetchall()
+        events_cur = await db.execute(
+            """
+            SELECT student_id, kind, COUNT(*) AS c
+            FROM student_events
+            WHERE sid = ? AND student_id IS NOT NULL
+            GROUP BY student_id, kind
+            """,
+            (sid,),
+        )
+        events = await events_cur.fetchall()
+    counts: dict[str, dict[str, int]] = {}
+    for row in events:
+        counts.setdefault(row["student_id"], {})[row["kind"]] = int(row["c"])
     for row in rows:
+        per = counts.get(row["student_id"], {})
         writer.writerow(
             [
                 row["sid"],
@@ -35,6 +63,9 @@ async def export_session_csv(db_path: str, sid: str) -> str:
                 "" if row["elapsed_ms"] is None else row["elapsed_ms"],
                 "" if row["score"] is None else row["score"],
                 row["status"] or "",
+                per.get("blur", 0),
+                per.get("visibility_hidden", 0),
+                per.get("duplicate_join", 0),
             ]
         )
     return out.getvalue()