feat: add model-availability validation to agentic insight generation (T009-T011)

- Verify custom model exists on at least one configured server before starting agentic loop; returns HTTP 400 with descriptive error if not found
- has_tool_calling field auto-serialised in GET /insights/models via existing ModelCapabilities Serialize derive
- model_version stored from OllamaClient.primary_model (already correct in T006 implementation)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ai/insight_generator.rs

@@ -1803,7 +1803,32 @@ Return ONLY the summary, nothing else."#,
 
         let insight_cx = current_cx.with_span(span);
 
-        // 2. Check tool calling capability
+        // 2a. Verify the model exists on at least one server before checking capabilities
+        if let Some(ref model_name) = custom_model {
+            let available_on_primary = OllamaClient::is_model_available(
+                &ollama_client.primary_url,
+                model_name,
+            )
+            .await
+            .unwrap_or(false);
+
+            let available_on_fallback = if let Some(ref fallback_url) = ollama_client.fallback_url {
+                OllamaClient::is_model_available(fallback_url, model_name)
+                    .await
+                    .unwrap_or(false)
+            } else {
+                false
+            };
+
+            if !available_on_primary && !available_on_fallback {
+                anyhow::bail!(
+                    "model not available: '{}' not found on any configured server",
+                    model_name
+                );
+            }
+        }
+
+        // 2b. Check tool calling capability
         let capabilities = OllamaClient::check_model_capabilities(
             &ollama_client.primary_url,
             &ollama_client.primary_model,