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/settings.rs

@@ -410,6 +410,8 @@ impl AnalysisSettings {
             grounding_prompt: prompt(&self.grounding_prompt, GROUNDING_PROMPT_DEFAULT),
             // Env-only secret preserved from the base.
             api_key: base.api_key.clone(),
+            // Env-only readiness probe URL preserved from the base.
+            vision_health_url: base.vision_health_url.clone(),
         })
     }
 }