feat(tv-power): implement server PR1 with tests and documentation

MQTT_EVENT_PAYLOAD_GUIDE.md

@@ -18,6 +18,36 @@ This document describes the MQTT message structure used by the Infoscreen system
 - **Format**: Integer (group ID)
 - **Purpose**: Assigns clients to groups
 
+### TV Power Intent (Phase 1)
+- **Topic**: `infoscreen/groups/{group_id}/power/intent`
+- **QoS**: 1
+- **Retained**: Yes
+- **Format**: JSON object
+- **Purpose**: Group-level desired power state for clients assigned to that group
+
+Phase 1 is group-only. Per-client power intent topics and client state/ack topics are deferred to Phase 2.
+
+Example payload:
+
+```json
+{
+  "schema_version": "tv-power-intent.v1",
+  "intent_id": "9cf26d9b-87a3-42f1-8446-e90bb6f6ce63",
+  "group_id": 12,
+  "desired_state": "on",
+  "reason": "active_event",
+  "issued_at": "2026-03-31T10:15:30Z",
+  "expires_at": "2026-03-31T10:17:00Z",
+  "poll_interval_sec": 30,
+  "source": "scheduler"
+}
+```
+
+Contract notes:
+- `intent_id` changes only on semantic transition (`desired_state`/`reason` changes).
+- Heartbeat republishes keep `intent_id` stable while refreshing `issued_at` and `expires_at`.
+- Expiry is poll-based: `max(3 x poll_interval_sec, 90)`.
+
 ## Message Structure
 
 ### General Principles