feat(telegram): add base_url support for custom Telegram Bot API server (#1021)

* feat(telegram): add base_url support for custom Telegram Bot API server

Allow users to specify a custom Telegram Bot API server URL via
config field `base_url` or env var `PICOCLAW_CHANNELS_TELEGRAM_BASE_URL`.
Defaults to the official https://api.telegram.org when left empty.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(telegram): trim whitespace and trailing slash from base_url

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

config/config.example.json

@@ -49,6 +49,7 @@
     "telegram": {
       "enabled": false,
       "token": "YOUR_TELEGRAM_BOT_TOKEN",
+      "base_url": "",
       "proxy": "",
       "allow_from": [
         "YOUR_USER_ID"