fix: auth redirect bug + cookie Secure + migration multi-statements

- fetchMe: handle ALL non-ok responses (not just 401) by calling tryRefresh
  → avoids user=null when backend returns 404/500/any error
- DOMContentLoaded guard: check isAuthenticated instead of localStorage token
  → immediate redirect if fetchMe+tryRefresh both fail, no more flash of dashboard
- Cookie Secure flag: check X-Forwarded-Proto header for Traefik/proxy setup
  → cookie gets Secure=true when behind TLS-terminating reverse proxy
- db.go migrate(): split SQL by ; and exec each statement separately
  → fixes SQLite multi-statement limitation (only first stmt was executed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/api/auth.go

@@ -116,12 +116,14 @@ func (h *AuthHandler) Login(w http.ResponseWriter, r *http.Request) {
 	h.db.Exec(`UPDATE users SET last_login_at = CURRENT_TIMESTAMP WHERE id = ?`, userID)
 
 	// Cookie httpOnly pour le refresh token
+	// Secure=true si TLS direct ou si derrière un proxy (Traefik) qui a terminé TLS
+	isHTTPS := r.TLS != nil || r.Header.Get("X-Forwarded-Proto") == "https"
 	http.SetCookie(w, &http.Cookie{
 		Name:     "pxp_refresh",
 		Value:    refreshToken,
 		Path:     "/api/auth/refresh",
 		HttpOnly: true,
-		Secure:   r.TLS != nil,
+		Secure:   isHTTPS,
 		SameSite: http.SameSiteStrictMode,
 		Expires:  expiry,
 	})