fix(telegram): preserve pre-#17686 chat-ID-in-_USERS configs + doc split

PR #15027 (5 days ago) shipped TELEGRAM_GROUP_ALLOWED_USERS as a chat-ID
allowlist. #17686 correctly renames that to sender user IDs and moves
chat IDs to TELEGRAM_GROUP_ALLOWED_CHATS. Without a shim, any user on
PR #15027's guidance would silently start rejecting group traffic on
upgrade.

- gateway/run.py: in _is_user_authorized, if TELEGRAM_GROUP_ALLOWED_USERS
  contains values starting with '-' (chat-ID-shaped), honor them as chat
  IDs and log a one-shot deprecation warning pointing users at the new
  TELEGRAM_GROUP_ALLOWED_CHATS var.
- tests/gateway/test_unauthorized_dm_behavior.py: three new tests cover
  legacy chat-ID values authorizing the listed chat, not crossing to
  other chats, and mixed sender/chat values in the same var.
- website/docs/user-guide/messaging/telegram.md: rewrite the Group
  Allowlisting section to document the new user/chat split + migration
  note. Remove stale '/thread_id' suffix claim (code never parsed it).
- website/docs/reference/environment-variables.md: document all three
  Telegram allowlist env vars.

gateway/run.py

@@ -3495,6 +3495,36 @@ class GatewayRunner:
             if "*" in allowed_group_ids or source.chat_id in allowed_group_ids:
                 return True
 
+        # Backward-compat shim for #15027: prior to PR #17686,
+        # TELEGRAM_GROUP_ALLOWED_USERS was (mis)used as a chat-ID allowlist.
+        # Values starting with "-" are Telegram chat IDs, not user IDs, so if
+        # users still have those in TELEGRAM_GROUP_ALLOWED_USERS we honor them
+        # as chat IDs and warn once. The correct var is now
+        # TELEGRAM_GROUP_ALLOWED_CHATS.
+        if (
+            source.platform == Platform.TELEGRAM
+            and group_user_allowlist
+            and source.chat_type in {"group", "forum"}
+            and source.chat_id
+        ):
+            legacy_chat_ids = {
+                v.strip()
+                for v in group_user_allowlist.split(",")
+                if v.strip().startswith("-")
+            }
+            if legacy_chat_ids:
+                if not getattr(self, "_warned_telegram_group_users_legacy", False):
+                    logger.warning(
+                        "TELEGRAM_GROUP_ALLOWED_USERS contains chat-ID-shaped values "
+                        "(%s). Treating them as chat IDs for backward compatibility. "
+                        "Move chat IDs to TELEGRAM_GROUP_ALLOWED_CHATS — the _USERS var "
+                        "is now for sender user IDs.",
+                        ",".join(sorted(legacy_chat_ids)),
+                    )
+                    self._warned_telegram_group_users_legacy = True
+                if source.chat_id in legacy_chat_ids:
+                    return True
+
         # Check if user is in any allowlist. In group/forum chats,
         # TELEGRAM_GROUP_ALLOWED_USERS is the scoped allowlist and should not
         # imply DM access; TELEGRAM_ALLOWED_USERS remains the platform-wide