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of a photo's insight (current plus superseded), newest-first, backing the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/ai/insight_chat.rs

@@ -2473,7 +2473,10 @@ mod tests {
         let original_len = msgs.len();
         let dropped = apply_context_budget(&mut msgs, budget_bytes);
 
-        assert!(!dropped, "short conversation with one image must not truncate");
+        assert!(
+            !dropped,
+            "short conversation with one image must not truncate"
+        );
         assert_eq!(msgs.len(), original_len, "no messages should be dropped");
         // Sanity: the flat image charge is accounted for but stays well under budget.
         assert!(estimate_bytes(&msgs) <= budget_bytes);