Add push-based wallet trade watcher (Alchemy webhook -> Discord)

webhook_receiver.py: zero-dep stdlib HTTP server that receives Alchemy
Address-Activity webhook POSTs, confirms/enriches the trade via Polymarket
data-API, and pushes a Discord alert — fires only on a real trade, no polling.
Cloud-ready: secrets via env (DISCORD_WEBHOOK, ALCHEMY_SIGNING_KEY), non-secret
watch list in committed watch.json, binds to $PORT, /health endpoint,
optional HMAC signature verification, in-memory dedup. Procfile for one-command
deploy. config.json (secrets) stays gitignored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -19,3 +19,5 @@ lp_paper_state.json
 # cross-venue scanner output
 xarb_hits.csv
 hunt.log
+recv.log
+watcher_state.json