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()

app/src/Database.php

@@ -2722,7 +2722,8 @@ class Database
                 t.duration_pages,
                 t.duration_minutes,
                 t.has_annexes,
-                t.objet
+                t.objet,
+                t.date_depot
             FROM theses t
             LEFT JOIN orientations    o  ON t.orientation_id  = o.id
             LEFT JOIN ap_programs     ap ON t.ap_program_id   = ap.id