refactor(timeline): extract shared resolve_entity_ref and make target_iid optional

The seed, expand, and collect stages each had their own near-identical
resolve_entity_ref helper that converted internal DB IDs to full EntityRef
structs. This duplication made it easy for bug fixes to land in one copy
but not the others.

Extract a single public resolve_entity_ref into timeline.rs with an
optional project_id parameter:
- Some(project_id): scopes the lookup (used by seed, which knows the
  project from the FTS result)
- None: unscoped lookup (used by expand, which traverses cross-project
  references)

Also changes UnresolvedRef.target_iid from i64 to Option<i64>. Cross-
project references parsed from descriptions may not always carry an IID
(e.g. when the reference is malformed or the target was deleted). The
previous sentinel value of 0 was semantically incorrect since GitLab IIDs
start at 1.

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

src/core/timeline.rs

@@ -1,7 +1,10 @@
 use std::cmp::Ordering;
 
+use rusqlite::Connection;
 use serde::Serialize;
 
+use super::error::Result;
+
 /// The core timeline event. All pipeline stages produce or consume these.
 /// Spec ref: Section 3.3 "Event Model"
 #[derive(Debug, Clone, Serialize)]
@@ -121,7 +124,7 @@ pub struct UnresolvedRef {
     pub source: EntityRef,
     pub target_project: Option<String>,
     pub target_type: String,
-    pub target_iid: i64,
+    pub target_iid: Option<i64>,
     pub reference_type: String,
 }
 
@@ -135,6 +138,45 @@ pub struct TimelineResult {
     pub unresolved_references: Vec<UnresolvedRef>,
 }
 
+/// Resolve an entity's internal DB id to a full [`EntityRef`] with iid and project path.
+///
+/// When `project_id` is `Some`, the query is scoped to that project.
+/// Returns `Ok(None)` for unknown entity types or when no matching row exists.
+pub fn resolve_entity_ref(
+    conn: &Connection,
+    entity_type: &str,
+    entity_id: i64,
+    project_id: Option<i64>,
+) -> Result<Option<EntityRef>> {
+    let table = match entity_type {
+        "issue" => "issues",
+        "merge_request" => "merge_requests",
+        _ => return Ok(None),
+    };
+
+    let sql = format!(
+        "SELECT e.iid, p.path_with_namespace
+         FROM {table} e
+         JOIN projects p ON p.id = e.project_id
+         WHERE e.id = ?1 AND (?2 IS NULL OR e.project_id = ?2)"
+    );
+
+    let result = conn.query_row(&sql, rusqlite::params![entity_id, project_id], |row| {
+        Ok((row.get::<_, i64>(0)?, row.get::<_, String>(1)?))
+    });
+
+    match result {
+        Ok((iid, project_path)) => Ok(Some(EntityRef {
+            entity_type: entity_type.to_owned(),
+            entity_id,
+            entity_iid: iid,
+            project_path,
+        })),
+        Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+        Err(e) => Err(e.into()),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;