fix: display db timestamps in Brussels time

(heure de dépôt was showing UTC)

- feat: add date_depot column (real TFE deposit date)
with CSV round-trip + Brussels→UTC sanitization
- fix: keep PHP default tz at UTC to preserve token/share-link
 expiry consistency; convert to Brussels only in db_datetime()

.ignore

@@ -3,3 +3,4 @@ nginx
 src/cache/rate_limit
 *.db-wal
 *.db-shm
+*.db.gz