Stage 16: connector test-env via UseTestEnvironment; pin it in the test contour

- bot.New now selects Telegram's test environment with the library's native
  tgbot.UseTestEnvironment() instead of a token += "/test" hack (functionally
  identical URL /bot<token>/test/METHOD, but idiomatic) + a bot test asserting
  the getMe path for both test and prod.
- ci.yaml pins TELEGRAM_TEST_ENV=true for the test contour (it IS the test
  environment) instead of a TEST_TELEGRAM_TEST_ENV variable: removes the
  confusing double-TEST, telegram-specific, prefixed operator knob and the
  secret-vs-variable footgun. Prod (Stage 17) leaves it false.
- deploy/README.md + PLAN.md updated.

.gitea/workflows/ci.yaml

@@ -175,7 +175,9 @@ jobs:
           TELEGRAM_MINIAPP_URL: ${{ vars.TEST_TELEGRAM_MINIAPP_URL }}
           TELEGRAM_GAME_CHANNEL_ID_EN: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID_EN }}
           TELEGRAM_GAME_CHANNEL_ID_RU: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID_RU }}
-          TELEGRAM_TEST_ENV: ${{ vars.TEST_TELEGRAM_TEST_ENV }}
+          # The test contour always uses Telegram's test environment — pinned here,
+          # not an operator variable. Stage 17's prod workflow leaves it false.
+          TELEGRAM_TEST_ENV: "true"
           VITE_TELEGRAM_BOT_ID: ${{ vars.TEST_VITE_TELEGRAM_BOT_ID }}
           VITE_TELEGRAM_LINK: ${{ vars.TEST_VITE_TELEGRAM_LINK }}
           VITE_GATEWAY_URL: ${{ vars.TEST_VITE_GATEWAY_URL }}