fix(auth): atomic refresh rotation and a 7-day absolute session limit

- Rotation claims the old token with a conditional update; of two
  concurrent refreshes only one gets a successor, so a stolen token cannot
  fork into two live sessions.
- A token rotated out less than 10 s ago (parallel tab) is refused without
  revoking every session; later reuse still revokes them all.
- Sessions end at most SESSION_MAX_AGE (default 7 days) after login: the
  login time is inherited by every rotated token and caps its expiry.
- The refresh cookie is cleared with the same attributes it was set with.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Backend/prisma/migrations/20261001110000_refresh_session_start/migration.sql

@@ -0,0 +1,3 @@
+-- Absolute session lifetime: every rotated refresh token keeps the time
+-- the session was first opened. Existing sessions count from now.
+ALTER TABLE "RefreshToken" ADD COLUMN "sessionStartedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP;