Tracing: carry span into insight panic-isolation tasks; root the watcher boot probe (#143)

Found by the parity capture: both insight-generation handlers run the
generation inside a nested tokio::task::spawn (panic isolation) that was
not instrumented, so the background ai.insight.* trace still split off
from its 202 request. The watcher's boot-time availability probe ran
before the first tick span, leaving its count_for_library DB spans as
roots; it now opens `watcher.initial_probe`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ai/handlers.rs

@@ -503,29 +503,33 @@ pub async fn generate_insight_handler(
 
             let path_for_task = path.clone();
             let generator_for_task = generator.clone();
-            let result = tokio::task::spawn(async move {
-                // Cross-model barrier: if a TTS synthesis holds the GPU, wait it
-                // out BEFORE the generation wall-clock starts. The per-request
-                // lease keeps reqwest budgets honest, but this job-level timeout
-                // would otherwise burn while the first chat call queues behind a
-                // multi-minute synthesis. Dropped immediately — holding it across
-                // the generation would deadlock the chat calls' own leases.
-                drop(crate::ai::gpu::llm_lease().await);
-                tokio::time::timeout(
-                    std::time::Duration::from_secs(timeout_secs),
-                    generator_for_task.generate_insight_for_photo_with_config(
-                        &path_for_task,
-                        request.model.clone(),
-                        request.system_prompt.clone(),
-                        request.num_ctx,
-                        request.temperature,
-                        request.top_p,
-                        request.top_k,
-                        request.min_p,
-                    ),
-                )
-                .await
-            })
+            let result = tokio::task::spawn(
+                async move {
+                    // Cross-model barrier: if a TTS synthesis holds the GPU, wait it
+                    // out BEFORE the generation wall-clock starts. The per-request
+                    // lease keeps reqwest budgets honest, but this job-level timeout
+                    // would otherwise burn while the first chat call queues behind a
+                    // multi-minute synthesis. Dropped immediately — holding it across
+                    // the generation would deadlock the chat calls' own leases.
+                    drop(crate::ai::gpu::llm_lease().await);
+                    tokio::time::timeout(
+                        std::time::Duration::from_secs(timeout_secs),
+                        generator_for_task.generate_insight_for_photo_with_config(
+                            &path_for_task,
+                            request.model.clone(),
+                            request.system_prompt.clone(),
+                            request.num_ctx,
+                            request.temperature,
+                            request.top_p,
+                            request.top_k,
+                            request.min_p,
+                        ),
+                    )
+                    .await
+                }
+                // Inner panic-isolation task: carry the job span in too.
+                .instrument(tracing::Span::current()),
+            )
             .await;
 
             let mut dao = job_dao.lock().expect("Unable to lock InsightJobDao");
@@ -883,32 +887,36 @@ pub async fn generate_agentic_insight_handler(
 
             let path_for_task = path.clone();
             let generator_for_task = generator.clone();
-            let result = tokio::task::spawn(async move {
-                // Cross-model barrier — see generate_insight_handler: wait out any
-                // running TTS synthesis before the generation wall-clock starts.
-                drop(crate::ai::gpu::llm_lease().await);
-                tokio::time::timeout(
-                    std::time::Duration::from_secs(timeout_secs),
-                    generator_for_task.generate_agentic_insight_for_photo(
-                        &path_for_task,
-                        request.model.clone(),
-                        system_prompt,
-                        request.num_ctx,
-                        request.temperature,
-                        request.top_p,
-                        request.top_k,
-                        request.min_p,
-                        request.enable_thinking,
-                        max_iterations,
-                        request.backend.clone(),
-                        fewshot_examples,
-                        fewshot_ids,
-                        user_id,
-                        persona_id,
-                    ),
-                )
-                .await
-            })
+            let result = tokio::task::spawn(
+                async move {
+                    // Cross-model barrier — see generate_insight_handler: wait out any
+                    // running TTS synthesis before the generation wall-clock starts.
+                    drop(crate::ai::gpu::llm_lease().await);
+                    tokio::time::timeout(
+                        std::time::Duration::from_secs(timeout_secs),
+                        generator_for_task.generate_agentic_insight_for_photo(
+                            &path_for_task,
+                            request.model.clone(),
+                            system_prompt,
+                            request.num_ctx,
+                            request.temperature,
+                            request.top_p,
+                            request.top_k,
+                            request.min_p,
+                            request.enable_thinking,
+                            max_iterations,
+                            request.backend.clone(),
+                            fewshot_examples,
+                            fewshot_ids,
+                            user_id,
+                            persona_id,
+                        ),
+                    )
+                    .await
+                }
+                // Inner panic-isolation task: carry the job span in too.
+                .instrument(tracing::Span::current()),
+            )
             .await;
 
             let mut dao = job_dao.lock().expect("Unable to lock InsightJobDao");