refactor!: rebrand miti99bot to tiennm99bot

BREAKING CHANGE: the Go module path is now github.com/tiennm99/tiennm99bot,
the default sticker pack is stickers_by_<bot username>, and the health body,
deploy DM, User-Agents and image names say tiennm99bot.

.env.example

@@ -1,4 +1,4 @@
-# miti99bot — self-host (Coolify + MongoDB Atlas) environment.
+# tiennm99bot — self-host (Coolify + MongoDB Atlas) environment.
 # Copy to .env and fill in. .env is gitignored — never commit real secrets.
 
 # ============================ Required ============================
@@ -9,7 +9,7 @@ TELEGRAM_BOT_TOKEN=123456:ABC-DEF...
 # db username + password — treat it as a secret (it is never logged).
 # Create a least-privilege user: readWrite on this one database only.
 MONGO_URL=mongodb+srv://botuser:STRONG_UNIQUE_PASSWORD@cluster0.xxxxx.mongodb.net/?retryWrites=true&w=majority
-MONGO_DATABASE=miti99bot
+MONGO_DATABASE=tiennm99bot
 
 # ============================ Operational =========================
 # Comma-separated module list. Empty = load every module, including any module
@@ -30,7 +30,7 @@ BOT_USERNAME=
 # 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 = miti99_by_<bot username>.
+# the first /addsticker if it does not exist yet. Unset = stickers_by_<bot username>.
 STICKER_PACK_NAME=
 
 # SOURCE_COMMIT (commit SHA) is read at startup for the deploynotify owner DM.