feat(019): discovery on its own thread + tuner heartbeat (honest stale alert)

Telemetry: discovery took 6-30s ON the single scheduler, freezing ingest/arm
so the trigger read stale bars — the real 'late edge detection'. Now discovery
runs on its own daemon thread with a dedicated DB connection; the watchlist it
mutates is a lock-guarded Watchlist (list-like drop-in, no torn reads); the
scheduler discover step auto-disables (single-writer). Loop is now 3 fast
threads (entries/exits/discovery) + slow scheduler for tolerant work.

Also: nightly_tune writes a heartbeat every run, so the boot alert distinguishes
'tuner not running' (real) from 'ran, no profitable ORB config, kept params'
(expected — the strategy has no edge). Kills the misleading Telegram alarm.
543 tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

deploy/docker/truenas-compose.yml

@@ -79,6 +79,7 @@ x-app: &app
     TRADING_EXIT_TRAIL_MODE: 'prior_low'
     TRADING_EXIT_MANAGE_INTERVAL_SECONDS: '4'  # 12->4s: react to fast small-cap round-trips (was giving back gains)
     TRADING_EXIT_THREAD_ENABLED: '1'
+    DISCOVER_THREAD_ENABLED: '1'
     TRADING_EXIT_THREAD_INTERVAL_SECONDS: '1.0'
     TRADING_EXIT_TRAIL_R_STEP: '0.25'
     TRADING_LIQUIDITY_MAX_VOLUME_PCT: '0.025'