docs(env): document TAPIR_PUBLIC_URL for tapir invite

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -51,3 +51,8 @@ TAPIR_POLL_INTERVAL=
 # caption endpoint; after it expires the video is retried. 0 = always retry.
 # Go duration; default 1h.
 TAPIR_FETCH_BACKOFF=
+
+# --- invitations (tapir invite) -------------------------------------------
+# Public base URL used to build the invite link `tapir invite <email>` prints.
+# Default https://tapir.d-ma.be; no trailing slash needed.
+TAPIR_PUBLIC_URL=