docs: add architecture.md with API + worker split overview

docs/architecture.md

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+# Architecture Overview
+
+PayStream is split into two binaries that communicate through a shared PostgreSQL database.
+
+## API (`cmd/paystream-api`)
+
+The HTTP API server listens on `:8080` and handles all synchronous client requests:
+
+- Payout batch creation and status queries
+- Recipient registration and payout method management
+- Schedule CRUD (recurring payroll runs)
+- Webhook endpoint registration
+- Health check at `GET /health`
+
+The API writes jobs to the database (e.g. pending payouts, scheduled runs) and returns immediately. It never submits Stellar transactions directly.
+
+## Worker (`cmd/paystream-worker`)
+
+The background worker processes jobs written by the API:
+
+- **Signing queue** — constructs and signs Stellar multi-operation transactions for pending payout batches
+- **Horizon polling** — monitors submitted transactions for settlement confirmation
+- **Webhook delivery** — emits events (`payout.completed`, etc.) to registered endpoints with HMAC-SHA256 signatures and exponential-backoff retries
+- **Schedule engine** — triggers recurring payroll runs on their CRON schedule
+
+## Communication
+
+```
+Client → API (HTTP :8080) → PostgreSQL ← Worker (background)
+                                       ↓
+                               Stellar Horizon (Testnet / Mainnet)
+                                       ↓
+                               Recipient webhooks (HTTPS)
+```
+
+The API and worker share no in-process state. Scaling either component independently is safe — run multiple API replicas behind a load balancer, and multiple worker replicas with advisory locks on job rows to prevent double-processing.
+
+## Key Internal Packages
+
+| Package | Responsibility |
+|---|---|
+| `internal/health` | Health check handler used by the API |
+| `internal/payout` | Batch chunking, Stellar transaction construction |
+| `internal/recipient` | Recipient CRUD and payout method storage |
+| `internal/schedule` | CRON-based recurring schedule engine |
+| `internal/webhook` | Event emission, HMAC signing, delivery with retries |
+| `internal/anchor` | SEP-31 corridor adapters (one file per anchor) |
+| `internal/compliance` | Audit log emission and SOC 2 evidence hooks |