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of a photo's insight (current plus superseded), newest-first, backing the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

@@ -25,7 +25,8 @@ pub use handlers::{
     chat_stream_handler, chat_turn_handler, delete_insight_handler, export_training_data_handler,
     generate_agentic_insight_handler, generate_insight_handler, generation_status_handler,
     get_all_insights_handler, get_available_models_handler, get_insight_handler,
-    get_openrouter_models_handler, rate_insight_handler, turn_async_handler, turn_replay_handler,
+    get_insight_history_handler, get_openrouter_models_handler, rate_insight_handler,
+    turn_async_handler, turn_replay_handler,
 };
 pub use insight_generator::InsightGenerator;
 pub use llamacpp::LlamaCppClient;