feat: YAML content importer + phys/ct-2024 collection (proof)

content/phys/ct-2024.yaml — 15 questions from ЦЭ,ЦТ 2024 across
6 topics (kinem, mol, emf, electro, magnet, optics) as proof of format.

backend/scripts/import-content.js — unified importer:
- Validates schema (subject, year, options, exactly-1-correct)
- Aliases (kinem, mol, ...) resolve to Russian topic names via get-or-create
- Deduplicates by first 80 chars of text (matches legacy seed_*.js behavior)
- Runs in a single transaction, idempotent re-runs

On fresh DB: 13 added (2 dedup collisions — same 80-char prefix, expected).
On prod DB: 0 added (all already exist from legacy seeds).
Second run on either: 0 added (dedup works).

Legacy seed_phys_ct2024.js kept as backup — see content/README.md
for migration guide.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/package-lock.json

@@ -13,6 +13,7 @@
         "cors": "^2.8.5",
         "dotenv": "^16.4.5",
         "express": "^4.18.3",
+        "js-yaml": "^4.1.1",
         "jsonwebtoken": "^9.0.2",
         "multer": "^2.1.1",
         "sharp": "^0.34.5",
@@ -530,6 +531,12 @@
       "integrity": "sha512-klpgFSWLW1ZEs8svjfb7g4qWY0YS5imI82dTg+QahUvJ8YqAY0P10Uk8tTyh9ZGuYEZEMaeJYCF5BFuX552hsw==",
       "license": "MIT"
     },
+    "node_modules/argparse": {
+      "version": "2.0.1",
+      "resolved": "https://registry.npmjs.org/argparse/-/argparse-2.0.1.tgz",
+      "integrity": "sha512-8+9WqebbFzpX9OR+Wa6O29asIogeRMzcGtAINdpMHHyAg10f05aSFVBbcEqGf/PXw1EjAZ+q2/bEBg3DvurK3Q==",
+      "license": "Python-2.0"
+    },
     "node_modules/array-flatten": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/array-flatten/-/array-flatten-1.1.1.tgz",
@@ -1255,6 +1262,18 @@
         "node": ">=0.12.0"
       }
     },
+    "node_modules/js-yaml": {
+      "version": "4.1.1",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.1.1.tgz",
+      "integrity": "sha512-qQKT4zQxXl8lLwBtHMWwaTcGfFOZviOJet3Oy/xmGk2gZH677CJM9EvtfdSkgWcATZhj/55JZ0rmy3myCT5lsA==",
+      "license": "MIT",
+      "dependencies": {
+        "argparse": "^2.0.1"
+      },
+      "bin": {
+        "js-yaml": "bin/js-yaml.js"
+      }
+    },
     "node_modules/jsonwebtoken": {
       "version": "9.0.3",
       "resolved": "https://registry.npmjs.org/jsonwebtoken/-/jsonwebtoken-9.0.3.tgz",