feat(stability): stage 6 — lightweight /ping healthcheck that never touches Telegram (#6)

The container healthcheck hit /rss/...?limit=1 (5s timeout): on a cold cache RSS
generation exceeds 5s, or a hung TG RPC makes it hang, so docker/autoheal restarts
the container mid-download and corrupts temp files. Replace it with /ping, which
reflects process/loop liveness (answers instantly, always) plus TG liveness read
from the watchdog's last-probe data — issuing ZERO Telegram RPC.

- telegram_client: public watchdog_last_ok_age() — seconds since the last successful
  watchdog probe (None if never). Pure read of the Stage-1 _wd_last_ok_monotonic
  field; no RPC.
- api_server: /ping route (no token, no TG RPC, no SQLite, no fs scan). healthy =
  connected and (age is None or age < threshold). age is None right after boot =>
  healthy (don't kill before the first probe). connected coerced to bool so the JSON
  "connected" field is always a bool (pre-start reports false, never null).
- config: TG_PING_UNHEALTHY_AFTER knob, default interval*(failures+1)+timeout = 250s
  (how long until the watchdog itself gives up), env-overridable.
- dockercompose.yml: healthcheck -> curl -sf http://127.0.0.1:80/ping, interval 5m,
  timeout 5s, retries 3, start_period 30s. Old /rss check removed, not left behind.
- tests: 10 (healthy/stale/disconnected/fresh-boot/pre-start-null/no-token +
  the anti-regression zero-TG-RPC spy across all branches + the accessor).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

telegram_client.py

@@ -259,6 +259,17 @@ class TelegramClient:
             # Emergency termination
             os._exit(1)
 
+    def watchdog_last_ok_age(self) -> float | None:
+        """Seconds since the last successful watchdog probe, or None if none succeeded yet.
+
+        Reads only the already-recorded monotonic timestamp set by the watchdog loop; it
+        never issues a Telegram RPC, so it is safe to call from the hot /ping path even
+        while a real RPC is hung.
+        """
+        if self._wd_last_ok_monotonic is None:
+            return None
+        return time.monotonic() - self._wd_last_ok_monotonic
+
     async def safe_get_messages(self, channel_id, post_id, max_retries=2):
         """Wrapper with retry logic for auth errors"""
         for attempt in range(max_retries):