Gateway cache policy and cache-instability handling

Never enable prompt caching for meta-request passthrough such as title generation or summaries; these requests lack the durable conversation-prefix contract. Keep cache-spiral diagnostics in Sentry rather than showing users an unactionable warning, while retaining visible worker-degradation warnings. For conversationTTL=auto, promote 5m to 1h immediately when cold-cache fraction exceeds 0.4, but require three consecutive turns below 0.2 before downgrading. Evaluate only after five samples and reset the downgrade streak after promotion or any nonqualifying turn.