feat(backend): auto-assign + transactional double-booking guard + working-hours slots

POST /book and POST /appointments now resolve the specialist via autoAssign (or isAvailable-guard a chosen one) inside BEGIN IMMEDIATE...COMMIT, returning 409 on conflict. Slots endpoint derives the day window from real working_hours and ranks the best specialist per slot. settings/employees expose + persist the new fields. Adds booking-e2e.mjs and adapts e2e-test.mjs fixtures to in-hours slots.

package.json

@@ -15,6 +15,8 @@
     "seed": "tsx server/scripts/seed.ts",
     "test:e2e": "node e2e-test.mjs",
     "test:admin": "node admin-test.mjs",
+    "test:unit": "node --import tsx --test server/lib/scheduling.test.ts",
+    "test:booking": "node server/scripts/booking-e2e.mjs",
     "audit:visual": "node visual-audit.mjs"
   },
   "dependencies": {