feat(scoring): rescale scores to 0.0-1.0 with 0.05 resolution

Per-question score is now a float in [0.0, 1.0] snapped to a 21-level
0.05 grid, replacing the previous 0-1000 integer scale. Easier to read
on a leaderboard, ties become acceptable rather than vanishingly rare,
and small clock-skew differences no longer split rankings.

DB schema: score is REAL now (SQLite type affinity is loose enough that
existing rows still read fine, but new inserts go in as floats).

Frontend: added fmtScore() helpers in admin.js and quiz.js to render
two decimal places consistently (0.85, 1.20, 5.00) so float-arithmetic
sums never display as 0.8500000000000001.

Tests: linear_decay/flat/exponential_decay assertions updated; added
a snap-to-grid invariant test.

static/admin.js

@@ -28,6 +28,13 @@ const store = {
 
 let countdownTimer = null;
 
+function fmtScore(value) {
+  // Scores are floats on a 0.05 grid in [0, 1]; sums can run up to N
+  // (one per question). Always render as fixed two-decimal so the
+  // leaderboard reads "0.85" / "1.20" / "5.00" cleanly.
+  return Number(value || 0).toFixed(2);
+}
+
 function escapeText(value) {
   return String(value ?? "").replace(/[&<>"']/g, (c) => ({
     "&": "&amp;",
@@ -377,7 +384,7 @@ function renderLeaderboardList(rows) {
         <li>
           <span class="rank">${r.rank}</span>
           <span class="who"><b>${escapeText(r.name)}</b>${r.student_id ? `<small>${escapeText(r.student_id)}</small>` : ""}</span>
-          <span class="score">${r.score}</span>
+          <span class="score">${fmtScore(r.score)}</span>
         </li>
       `).join("")}
     </ol>