refactor(rate-limit): 1 req/sec cooldown, batch up to 2048 (#3)

* refactor(rate-limit): switch to 1 req/sec cooldown, batch size up to 2048

Replace per-pixel credit/token-bucket model with a simple per-user cooldown
(SET NX EX 1). Batch size is now independent of the rate limit and capped
at MAX_BATCH_SIZE = 2048.

- rate-limiter: SET NX EX replaces Lua credit script
- worker: response shape { ok: true } (no credits field)
- client: drop credit state/timer/UserInfo; uploader paces by cooldown
- tests: mock checkRateLimit; integration test exercises SET NX EX
- docs: README, system-architecture, code-standards, deployment-guide

* chore(plans): remove implemented plan directories

rplace-implementation (base build), review-fixes, and
image-importer-enhancements are all shipped. Keep plans/reports/ as
historical code-review and research references.

docs/deployment-guide.md

@@ -62,7 +62,7 @@ npx wrangler domains add rplace.yourdomain.com
 |---|---|---|
 | CF Workers | 100K requests/day | Canvas reads + pixel placements |
 | CF Durable Objects | Free with Workers | WebSocket connections |
-| Upstash Redis | 10K commands/day | BITFIELD reads/writes + rate limiting |
+| Upstash Redis | 10K commands/day | BITFIELD reads/writes + cooldown SET NX EX |
 
 For hobby traffic (< few hundred users/day), free tiers are sufficient. Upstash pay-as-you-go ($0.2/100K commands) is the first thing to hit limits.
 
@@ -71,4 +71,4 @@ For hobby traffic (< few hundred users/day), free tiers are sufficient. Upstash
 - **Canvas loads empty**: Check Upstash credentials in secrets
 - **Pixels don't persist**: Verify BITFIELD support — test with `redis-cli BITFIELD canvas SET u5 #0 1`
 - **WebSocket not connecting**: Ensure Durable Object migration ran (check `wrangler.json` migrations)
-- **Rate limiting not working**: Verify `redis.eval()` works on your Upstash tier (Lua scripting)
+- **Rate limiting not working**: Verify `SET key value NX EX 1` returns `"OK"` / `null` as expected on your Upstash tier