Require secrets via headers only: drop ?key= and ?password= query fallbacks (#137, #141)

Query strings leak into Traefik access logs, browser history, and Referer
headers. Admin key is now accepted only via X-Admin-Key; paste and can
passwords only via X-Paste-Password (or the POST unlock form). Tests
updated; new negative cases assert 401 for the query paths.

internal/api/admin_test.go

@@ -37,11 +37,12 @@ func TestAdminAuth(t *testing.T) {
 		t.Fatalf("wrong key: expected 401, got %d", rec.Code)
 	}
 
+	// #137: the ?key= query fallback was removed; keys must go via header.
 	req = httptest.NewRequest("GET", "/admin/api/settings?key=test-admin-key", nil)
 	rec = httptest.NewRecorder()
 	h.ServeHTTP(rec, req)
-	if rec.Code != 200 {
-		t.Fatalf("query key: expected 200, got %d", rec.Code)
+	if rec.Code != 401 {
+		t.Fatalf("query key: expected 401 after #137 removal, got %d", rec.Code)
 	}
 
 	req = httptest.NewRequest("GET", "/admin/api/settings", nil)