Add per-file insight history endpoint and rate-by-id

Expose GET /insights/history?path=... returning every generated version
of a photo's insight (current plus superseded), newest-first, backing the
mobile per-file insight history view.

- New get_insight_history_handler; reuses the existing get_insight_history
  DAO method (removed its dead_code allow).
- impl From<PhotoInsight> for PhotoInsightResponse, collapsing the mapping
  that was duplicated across the single-get and all-insights handlers.
- rate_insight_by_id DAO method + optional insight_id on RateInsightRequest
  so previously generated versions can be approved/rejected (the path-based
  rate only touches the current row).
- DAO tests for history ordering/scoping and id-targeted rating.
- cargo fmt normalized a multi-line assert in insight_chat.rs tests.

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

src/main.rs

@@ -351,6 +351,7 @@ fn main() -> std::io::Result<()> {
                 .service(ai::get_insight_handler)
                 .service(ai::delete_insight_handler)
                 .service(ai::get_all_insights_handler)
+                .service(ai::get_insight_history_handler)
                 .service(ai::get_available_models_handler)
                 .service(ai::get_openrouter_models_handler)
                 .service(ai::chat_turn_handler)