feat: refresh data from baotintuc.vn source, fix overflow sheet loss

Dataset update:
- Crawl all 63 .xls province files from baotintuc.vn CDN (original source)
- Old xlsx dataset moved to data-old/ for reference
- Net: +13,719 students (Hà Nội +7,275, HCM +6,445) — the old .xls → xlsx
  conversion silently dropped rows beyond the 65,536 per-sheet cap
- Also removes 1 bogus header row that had leaked into the old DB
- 100% identical scores on the 847,348 SBDs present in both datasets

Build pipeline:
- build-database.js: iterate ALL sheets per workbook (fixes the overflow
  loss) and accept .xls in addition to .xlsx

Audit tooling:
- scripts/crawl-baotintuc.js: idempotent 63-province downloader
- scripts/diff-datasets.js: compares two DBs by SBD set and per-column
  score deltas

scripts/diff-datasets.js

@@ -0,0 +1,113 @@
+// Compare new DB (public/thptqg2017.db) vs backup old DB (backup/thptqg2017.old.db)
+import Database from "better-sqlite3";
+
+const NEW = new Database("public/thptqg2017.db", { readonly: true });
+const OLD = new Database("backup/thptqg2017.old.db", { readonly: true });
+
+const SCORE_COLS = [
+  "toan", "ngu_van", "vat_ly", "hoa_hoc", "sinh_hoc", "khtn",
+  "lich_su", "dia_ly", "gdcd", "khxh",
+  "tieng_anh", "tieng_phap", "tieng_nga", "tieng_trung",
+];
+
+// Old schema may lack some newer columns — detect and project a subset
+const oldCols = new Set(
+  OLD.prepare("PRAGMA table_info(student)").all().map((c) => c.name),
+);
+const commonCols = SCORE_COLS.filter((c) => oldCols.has(c));
+
+const newCount = NEW.prepare("SELECT COUNT(*) c FROM student").get().c;
+const oldCount = OLD.prepare("SELECT COUNT(*) c FROM student").get().c;
+console.log(`\n=== Row counts ===`);
+console.log(`  new: ${newCount}`);
+console.log(`  old: ${oldCount}`);
+console.log(`  diff: ${newCount - oldCount >= 0 ? "+" : ""}${newCount - oldCount}`);
+
+// Build SBD sets (memory: 861k strings ~50 MB, fine)
+const newSbd = new Set(
+  NEW.prepare("SELECT so_bao_danh FROM student").all().map((r) => r.so_bao_danh),
+);
+const oldSbd = new Set(
+  OLD.prepare("SELECT so_bao_danh FROM student").all().map((r) => r.so_bao_danh),
+);
+
+let newOnly = 0, oldOnly = 0, common = 0;
+for (const s of newSbd) (oldSbd.has(s) ? common++ : newOnly++);
+for (const s of oldSbd) if (!newSbd.has(s)) oldOnly++;
+
+console.log(`\n=== SBD membership ===`);
+console.log(`  new-only (added):     ${newOnly}`);
+console.log(`  old-only (removed):   ${oldOnly}`);
+console.log(`  common:               ${common}`);
+
+// Sample 5 new-only SBDs to peek
+const sampleNewOnly = [];
+for (const s of newSbd) {
+  if (!oldSbd.has(s)) sampleNewOnly.push(s);
+  if (sampleNewOnly.length >= 5) break;
+}
+if (sampleNewOnly.length) {
+  const rows = NEW.prepare(
+    `SELECT so_bao_danh, ho_ten FROM student WHERE so_bao_danh IN (${sampleNewOnly.map(() => "?").join(",")})`,
+  ).all(...sampleNewOnly);
+  console.log(`  sample new-only rows:`);
+  rows.forEach((r) => console.log(`    ${r.so_bao_danh}  ${r.ho_ten}`));
+}
+
+// Score comparison on common SBDs
+console.log(`\n=== Score comparison (common SBDs) ===`);
+console.log(`  columns compared: ${commonCols.join(", ")}`);
+
+const newStmt = NEW.prepare(
+  `SELECT so_bao_danh, ${commonCols.join(",")} FROM student`,
+);
+const oldStmt = OLD.prepare(
+  `SELECT so_bao_danh, ${commonCols.join(",")} FROM student`,
+);
+
+// Build old map
+const oldMap = new Map();
+for (const r of oldStmt.iterate()) oldMap.set(r.so_bao_danh, r);
+
+let identical = 0,
+  differ = 0;
+const colChanges = Object.fromEntries(commonCols.map((c) => [c, { changed: 0, newNull: 0, oldNull: 0 }]));
+const sampleDiffs = [];
+
+for (const nr of newStmt.iterate()) {
+  const or = oldMap.get(nr.so_bao_danh);
+  if (!or) continue;
+  let rowDiffers = false;
+  const rowChanges = [];
+  for (const c of commonCols) {
+    const a = nr[c], b = or[c];
+    if (a === b) continue;
+    // treat very-close floats as equal (xlsx may emit 5.5 vs 5.50)
+    if (a !== null && b !== null && Math.abs(a - b) < 1e-9) continue;
+    rowDiffers = true;
+    colChanges[c].changed++;
+    if (a === null) colChanges[c].newNull++;
+    if (b === null) colChanges[c].oldNull++;
+    rowChanges.push(`${c}: ${b} → ${a}`);
+  }
+  if (rowDiffers) {
+    differ++;
+    if (sampleDiffs.length < 5) sampleDiffs.push({ sbd: nr.so_bao_danh, changes: rowChanges });
+  } else {
+    identical++;
+  }
+}
+
+console.log(`  identical:  ${identical}`);
+console.log(`  differing:  ${differ}`);
+console.log(`  per-column changes:`);
+for (const [c, info] of Object.entries(colChanges)) {
+  if (info.changed === 0) continue;
+  console.log(`    ${c.padEnd(14)} changed=${info.changed}  (new-null=${info.newNull}, old-null=${info.oldNull})`);
+}
+if (sampleDiffs.length) {
+  console.log(`  sample diffs:`);
+  sampleDiffs.forEach((d) => {
+    console.log(`    SBD ${d.sbd}: ${d.changes.slice(0, 4).join("; ")}${d.changes.length > 4 ? " ..." : ""}`);
+  });
+}