feat(ai): chat continuation for photo insights (server v1)

Adds POST /insights/chat and GET /insights/chat/history. Replays the
stored agentic conversation through the same backend the insight was
generated with (or a per-turn override), runs a short tool-calling
loop, and persists the extended history in append or amend mode.

Backend switching: same-backend or hybrid->local replay verbatim;
local->hybrid is rejected in v1 (would require on-the-fly vision
description rewrite).

Per-(library, file) async mutex serialises concurrent turns. Soft
context budget drops oldest tool_call+result pairs when the
serialized history exceeds num_ctx - 2048 tokens.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -1356,6 +1356,8 @@ fn main() -> std::io::Result<()> {
                 .service(ai::get_all_insights_handler)
                 .service(ai::get_available_models_handler)
                 .service(ai::get_openrouter_models_handler)
+                .service(ai::chat_turn_handler)
+                .service(ai::chat_history_handler)
                 .service(ai::rate_insight_handler)
                 .service(ai::export_training_data_handler)
                 .service(libraries::list_libraries)