fix: Accept string instance_id in telemetry endpoint

The orchestrator sends instance_id as a string (e.g., "letsbe-orchestrator")
but the endpoint was expecting a UUID path parameter. This caused 422
validation errors when orchestrators tried to send telemetry.

- Changed path parameter from UUID to str
- Lookup instance by Instance.instance_id (string) instead of Instance.id (UUID)
- Store telemetry with instance.id (UUID) for correct FK relationship
- Updated TelemetryPayload schema to use str instead of UUID

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/telemetry.py

@@ -7,7 +7,6 @@ It validates authentication, stores metrics, and updates instance state.
 import hashlib
 import logging
 import secrets
-from uuid import UUID
 
 from fastapi import APIRouter, Header, HTTPException, status
 from sqlalchemy import select
@@ -27,7 +26,7 @@ router = APIRouter(prefix="/api/v1/instances", tags=["Telemetry"])
 
 @router.post("/{instance_id}/telemetry", response_model=TelemetryResponse)
 async def receive_telemetry(
-    instance_id: UUID,
+    instance_id: str,
     payload: TelemetryPayload,
     db: AsyncSessionDep,
     hub_api_key: str = Header(..., alias="X-Hub-Api-Key"),
@@ -64,8 +63,8 @@ async def receive_telemetry(
             },
         )
 
-    # Find instance by UUID (id column, not instance_id string)
-    result = await db.execute(select(Instance).where(Instance.id == instance_id))
+    # Find instance by instance_id string (e.g., "letsbe-orchestrator")
+    result = await db.execute(select(Instance).where(Instance.instance_id == instance_id))
     instance = result.scalar_one_or_none()
 
     if instance is None:
@@ -114,8 +113,9 @@ async def receive_telemetry(
 
     # Store telemetry sample
     # Use PostgreSQL upsert to handle duplicates gracefully
+    # Note: instance_id in DB is the UUID (instance.id), not the string instance_id
     telemetry_data = {
-        "instance_id": instance_id,
+        "instance_id": instance.id,
         "window_start": payload.window_start,
         "window_end": payload.window_end,
         "uptime_seconds": payload.uptime_seconds,