Add README with architecture overview, SSM parameters, and phone setup instructions

README.md

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+# Sea Haven Door Unlock API
+
+AWS Lambda middleware that allows a Yealink T58A desk phone to unlock the front door controlled by LenelS2 Elements. Press a DSS key on the phone, and the door unlocks.
+
+```
+Yealink T58A → HTTPS GET → API Gateway → Lambda → LenelS2 Elements API → Door Unlocks
+```
+
+## Architecture
+
+- **API Gateway (HTTP API)** — single `GET /unlock` endpoint with throttling (5 burst / 2 sustained req/sec)
+- **Lambda (Node.js 20.x)** — validates a shared auth token, calls the Elements `TemporaryUnlock` command
+- **SSM Parameter Store** — stores the Elements API key, auth token, and door device ID
+- **Custom Domain** — `doorunlock.seahaven.com` via Route 53 + ACM wildcard cert
+
+## SSM Parameters
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `/seahaven/door-unlock/elements-api-key` | SecureString | LenelS2 Elements API key |
+| `/seahaven/door-unlock/auth-token` | SecureString | Shared secret embedded in the Yealink DSS key URL |
+| `/seahaven/door-unlock/door-id` | String | Elements device ID for the front door reader |
+
+## Deployment
+
+```bash
+npm install
+npx cdk deploy
+```
+
+## Phone Configuration
+
+Configure a DSS key on the Yealink T58A (via phone web UI or 3CX):
+
+- **Type:** URL
+- **Label:** Unlock Door
+- **Value:** `https://doorunlock.seahaven.com/unlock?token=<auth-token-value>`