fix(cron): route Telegram cron deliveries to a dedicated topic via TELEGRAM_CRON_THREAD_ID

When Telegram topic mode is enabled, cron messages delivered to the bot's
root DM (TELEGRAM_HOME_CHANNEL without a thread id) land in the system
lobby — replies there are rebuffed with the lobby reminder and
reply_to_message_id is dropped, so users cannot interact with the cron
output (#24409).

Add an optional TELEGRAM_CRON_THREAD_ID env var that overrides
TELEGRAM_HOME_CHANNEL_THREAD_ID for cron deliveries only. Operators can
create a "Cron" forum topic in the DM, point this var at its thread id,
and replies to cron messages will land in that topic's existing session
instead of the lobby. The home-channel thread id (used elsewhere, e.g.
restart notifications) is unchanged, and explicit
deliver="telegram:chat:thread" targets continue to win over the env var.

Per the reporter's clarification on 2026-05-13, option (a) (cron-side
route to a dedicated topic + config knob) was chosen.

Fixes #24409

.env.example

@@ -339,6 +339,7 @@ BROWSER_INACTIVITY_TIMEOUT=120
 # TELEGRAM_ALLOWED_USERS=                  # Comma-separated user IDs
 # TELEGRAM_HOME_CHANNEL=                   # Default chat for cron delivery
 # TELEGRAM_HOME_CHANNEL_NAME=              # Display name for home channel
+# TELEGRAM_CRON_THREAD_ID=                 # Forum topic ID for cron deliveries; overrides TELEGRAM_HOME_CHANNEL_THREAD_ID for cron so replies work in topic mode
 
 # Webhook mode (optional — for cloud deployments like Fly.io/Railway)
 # Default is long polling. Setting TELEGRAM_WEBHOOK_URL switches to webhook mode.