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/database/insights_dao.rs

@@ -60,6 +60,17 @@ pub trait InsightDao: Sync + Send {
         &mut self,
         context: &opentelemetry::Context,
     ) -> Result<Vec<PhotoInsight>, DbError>;
+
+    /// Replace the `training_messages` JSON blob on the current row for
+    /// `(library_id, rel_path)`. Used by chat-turn append mode to persist
+    /// the extended conversation without inserting a new insight version.
+    fn update_training_messages(
+        &mut self,
+        context: &opentelemetry::Context,
+        library_id: i32,
+        file_path: &str,
+        training_messages_json: &str,
+    ) -> Result<(), DbError>;
 }
 
 pub struct SqliteInsightDao {
@@ -265,4 +276,30 @@ impl InsightDao for SqliteInsightDao {
         })
         .map_err(|_| DbError::new(DbErrorKind::QueryError))
     }
+
+    fn update_training_messages(
+        &mut self,
+        context: &opentelemetry::Context,
+        lib_id: i32,
+        path: &str,
+        training_messages_json: &str,
+    ) -> Result<(), DbError> {
+        trace_db_call(context, "update", "update_training_messages", |_span| {
+            use schema::photo_insights::dsl::*;
+
+            let mut connection = self.connection.lock().expect("Unable to get InsightDao");
+
+            diesel::update(
+                photo_insights
+                    .filter(library_id.eq(lib_id))
+                    .filter(rel_path.eq(path))
+                    .filter(is_current.eq(true)),
+            )
+            .set(training_messages.eq(Some(training_messages_json.to_string())))
+            .execute(connection.deref_mut())
+            .map(|_| ())
+            .map_err(|_| anyhow::anyhow!("Update error"))
+        })
+        .map_err(|_| DbError::new(DbErrorKind::UpdateError))
+    }
 }