feat(config): resolve the bot username from BOT_USERNAME or getMe

The default sticker pack name and the lol User-Agent now follow whichever
bot runs the code, so a bot account change needs no code edit. The default
pack becomes miti99_by_<bot username>.

.env.example

@@ -21,12 +21,17 @@ OWNER_ID=
 # Comma-separated admin Telegram user ids (renamed from ADMIN_USER_IDS).
 ADMIN_IDS=
 
+# Bot's Telegram username, without "@". Unset = asked from Telegram (getMe)
+# once at startup. Used for the default sticker pack name and the lol
+# User-Agent. Set it to skip that call or to pin the value.
+BOT_USERNAME=
+
 # Sticker set /addsticker writes to. Every user contributes to this one pack.
 # MUST end in _by_<this bot username>: Telegram requires that suffix on sets a
 # bot creates and refuses to edit sets it did not create, so the suffix is what
 # proves the pack is manageable. Created automatically, owned by OWNER_ID, on
-# the first /addsticker if it does not exist yet. Unset = the default below.
-STICKER_PACK_NAME=miti99_by_miti99bot
+# the first /addsticker if it does not exist yet. Unset = miti99_by_<bot username>.
+STICKER_PACK_NAME=
 
 # SOURCE_COMMIT (commit SHA) is read at startup for the deploynotify owner DM.
 # Do NOT set it here. Coolify provides it at runtime. Keep "Include Source