feat(analysis): gate page leasing on a vision readiness probe

The analysis worker leases an analyze_page job (incrementing attempts in
SQL) before calling vision, so a stopped/loading vision burns all retries
and writes a permanent failed page_analysis row. With the vision-manager
autoscaler idle-stopping the container, that would poison the queue on
every cold start.

Add an optional VisionReadiness seam (HTTP GET /health in production) wired
via the new env-only ANALYSIS_VISION_HEALTH_URL. When set, the worker parks
without leasing until the probe answers 2xx; jobs stay pending with
attempts untouched and resume the instant vision is ready. None preserves
today's behavior for always-on endpoints.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/app.rs

@@ -420,6 +420,24 @@ fn spawn_analysis_daemon(
         model: cfg.model.clone(),
         max_image_bytes: cfg.max_image_bytes,
     });
+    // When a readiness URL is configured, gate leasing on it so an
+    // autoscaler that idle-stops the vision container never lets a job burn
+    // its retries. A dedicated short-timeout client keeps the probe snappy
+    // and independent of the (long) per-request analysis timeout.
+    let readiness: Option<Arc<dyn crate::analysis::daemon::VisionReadiness>> =
+        match &cfg.vision_health_url {
+            Some(url) if !url.is_empty() => {
+                let probe = reqwest::Client::builder()
+                    .timeout(std::time::Duration::from_secs(5))
+                    .build()
+                    .context("build vision readiness http client")?;
+                Some(Arc::new(crate::analysis::daemon::HttpVisionReadiness {
+                    http: probe,
+                    health_url: url.clone(),
+                }))
+            }
+            _ => None,
+        };
     let handle = crate::analysis::daemon::spawn(
         db,
         CancellationToken::new(),
@@ -428,6 +446,7 @@ fn spawn_analysis_daemon(
             workers: cfg.workers,
             job_timeout: cfg.job_timeout,
             events,
+            readiness,
         },
     );
     tracing::info!(workers = cfg.workers, model = %cfg.model, "analysis worker daemon started");