feat(insights): allow AI observations to link directly to devices

The LLM can now copy device_id from the snapshot into its observation
output. The worker wires it into the insight's device_id FK so the UI
can link directly to the referenced device.

lib/towerops/llm/network_insight_prompt.ex

@@ -53,7 +53,8 @@ defmodule Towerops.LLM.NetworkInsightPrompt do
   Respond ONLY with JSON of this shape:
   {"observations": [
     {"title": "...", "description": "...", "urgency": "critical"|"warning"|"info",
-     "recommended_action": "..."}
+     "recommended_action": "...",
+     "device_id": "..." (optional — copy the exact device_id from the snapshot for any device you reference)}
   ]}
 
   Aim for 0–5 observations. Quality over quantity.

lib/towerops/workers/ai_network_insight_worker.ex

@@ -91,7 +91,8 @@ defmodule Towerops.Workers.AiNetworkInsightWorker do
       llm_summary: obs.description,
       llm_model: model,
       llm_enriched_at: now,
-      metadata: metadata
+      metadata: metadata,
+      device_id: Map.get(obs.metadata, "device_id")
     }
 
     case Insights.insert_insight_if_new(attrs) do