feat(bot): report Telegram bot Bot API health to the gateway over the bot-link

The bot runs on its own host and exports no telemetry (otelcol is unreachable
from there), so it was a monitoring blind spot. Observe its Bot API health
centrally by wrapping the HTTP client — one place, no per-call-site
instrumentation — and relay it up the existing bot-link as a periodic Health
message the gateway turns into its own metrics.

- proto: add Health (delta connect / api / 429 counters + a last-ok stamp) to
  the FromBot oneof (additive, backward-compatible).
- bot: platform/telegram/internal/health wraps the Bot API HTTP client — it
  stamps liveness on any 2xx (so the getUpdates long-poll keeps it fresh even
  when idle), classifies transport/5xx failures (getUpdates vs other) and 429s,
  and honours a 429's Retry-After (bounded) so the bot backs off; a 4xx other
  than 429 is a normal per-request outcome and is not counted.
- bot-link client: flush the reporter as a Health message every 30s over a
  single-sender loop (a gRPC stream forbids concurrent Send).
- gateway: fold each report into bot_tg_errors_total{kind} and the
  bot_tg_last_ok_unix liveness gauge.
- grafana: alerts (bot disconnected, bot not reaching the Bot API, sustained
  429s) routed to the operator email — which does not go through the bot — plus
  a Telegram-bot dashboard.
- docs (ARCHITECTURE, compose comments); unit tests (observer classification,
  Retry-After, snapshot/commit, hub last-ok monotonicity).

deploy/docker-compose.bot.yml

@@ -3,8 +3,10 @@
 #   docker compose -f docker-compose.bot.yml up -d
 #
 # The bot egresses to the Bot API directly (no VPN sidecar) and dials the main host's
-# published bot-link :9443 over mTLS. It exports no telemetry — otelcol lives on the
-# main host and is unreachable from here — so observe it via `docker logs` on this host.
+# published bot-link :9443 over mTLS. It exports no OTLP telemetry — otelcol lives on the
+# main host and is unreachable from here — but it reports its Bot API health up the bot-link,
+# which the gateway turns into metrics + alerts on the main host (docs/ARCHITECTURE.md); `docker
+# logs` on this host is the local detail view.
 # Values come from the prod-deploy workflow (PROD_ secrets/variables); BOT_IMAGE is the
 # pushed registry tag and BOTLINK_GATEWAY_ADDR is the main host's <ip>:9443.
 name: scrabble-bot
@@ -50,7 +52,8 @@ services:
       TELEGRAM_BOTLINK_TLS_CA: /certs/ca.crt
       TELEGRAM_LOG_LEVEL: ${LOG_LEVEL:-info}
       TELEGRAM_SERVICE_NAME: scrabble-telegram-bot
-      # No telemetry export: otelcol is on the main host, unreachable from here.
+      # No OTLP export: otelcol is on the main host, unreachable from here. Bot API health rides the
+      # bot-link instead (the gateway exposes it as metrics); see docs/ARCHITECTURE.md.
       TELEGRAM_OTEL_TRACES_EXPORTER: none
       TELEGRAM_OTEL_METRICS_EXPORTER: none
       GOMAXPROCS: "1"