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.go

@@ -160,13 +160,12 @@ func HandleResetAdminKey(dbPath string) {
 }
 
 // adminKeyOK reports whether the request carries the correct admin key via
-// X-Admin-Key header or ?key=. Constant-time compare; failures and successes
-// are both logged (#40).
+// the X-Admin-Key header only. The ?key= query fallback was removed (#137):
+// query strings land in access logs, browser history, and Referer headers,
+// so accepting the key there leaked the admin secret. Constant-time compare;
+// failures and successes are both logged (#40).
 func (a *apiServer) adminKeyOK(r *http.Request, key string) bool {
 	given := r.Header.Get("X-Admin-Key")
-	if given == "" {
-		given = r.URL.Query().Get("key")
-	}
 	return subtle.ConstantTimeCompare([]byte(given), []byte(key)) == 1
 }