perf(timeline): hoist prepared statement outside discussion thread loop

Moves the conn.prepare() call for fetching discussion notes outside the
per-discussion loop in collect_discussion_threads(). The SQL is identical
for every iteration, so preparing it once and rebinding parameters avoids
redundant statement compilation on each matched discussion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/timeline_collect.rs

@@ -411,6 +411,12 @@ fn collect_discussion_threads(
     // Deduplicate by discussion_id
     let mut seen = HashSet::new();
 
+    let mut stmt = conn.prepare(
+        "SELECT id, author_username, body, created_at FROM notes
+         WHERE discussion_id = ?1 AND is_system = 0
+         ORDER BY created_at ASC",
+    )?;
+
     for disc in matched_discussions {
         if !seen.insert(disc.discussion_id) {
             continue;
@@ -422,12 +428,6 @@ fn collect_discussion_threads(
                 None => continue, // entity not in seed or expanded set
             };
 
-        let mut stmt = conn.prepare(
-            "SELECT id, author_username, body, created_at FROM notes
-             WHERE discussion_id = ?1 AND is_system = 0
-             ORDER BY created_at ASC",
-        )?;
-
         let rows = stmt.query_map(rusqlite::params![disc.discussion_id], |row| {
             Ok((
                 row.get::<_, i64>(0)?,            // id