Make auth cookie SameSite configurable

Add same-site to auth cookie config with lax as the default,
PROBOD_AUTH_COOKIE_SAMESITE bootstrap mapping, and validation
that rejects none unless Secure is enabled.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

.env.example

@@ -47,6 +47,7 @@
 # ── Cookie ────────────────────────────────────────────────────────────
 # PROBOD_AUTH_COOKIE_DOMAIN=localhost
 # PROBOD_AUTH_COOKIE_SECURE=false
+# PROBOD_AUTH_COOKIE_SAMESITE=lax
 # PROBOD_AUTH_COOKIE_DURATION=24
 
 # ── Postgres ──────────────────────────────────────────────────────────