feat(transformers): Add MR transformer and polymorphic discussion support

Introduces NormalizedMergeRequest transformer and updates discussion
normalization to handle both issue and MR discussions polymorphically.

New transformers:
- NormalizedMergeRequest: Transforms API MergeRequest to database row,
  extracting labels/assignees/reviewers into separate collections for
  junction table insertion. Handles draft detection, detailed_merge_status
  preference over deprecated merge_status, and merge_user over merged_by.

Discussion transformer updates:
- NormalizedDiscussion now takes noteable_type ("Issue" | "MergeRequest")
  and noteable_id for polymorphic FK binding
- normalize_discussions_for_issue(): Convenience wrapper for issues
- normalize_discussions_for_mr(): Convenience wrapper for MRs
- DiffNote position fields (type, line_range, SHA triplet) now extracted
  from API position object for code review context

Design decisions:
- Transformer returns (normalized_item, labels, assignees, reviewers)
  tuple for efficient batch insertion without re-querying
- Timestamps converted to ms epoch for SQLite storage consistency
- Optional fields use map() chains for clean null handling

The polymorphic discussion approach allows reusing the same discussions
and notes tables for both issues and MRs, with noteable_type + FK
determining the parent relationship.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitlab/transformers/discussion.rs

@@ -46,6 +46,18 @@ pub struct NormalizedNote {
     pub resolved: bool,
     pub resolved_by: Option<String>,
     pub resolved_at: Option<i64>,
+    // DiffNote position fields (CP1 - basic path/line)
+    pub position_old_path: Option<String>,
+    pub position_new_path: Option<String>,
+    pub position_old_line: Option<i32>,
+    pub position_new_line: Option<i32>,
+    // DiffNote extended position fields (CP2)
+    pub position_type: Option<String>, // "text" | "image" | "file"
+    pub position_line_range_start: Option<i32>, // multi-line comment start
+    pub position_line_range_end: Option<i32>, // multi-line comment end
+    pub position_base_sha: Option<String>, // Base commit SHA for diff
+    pub position_start_sha: Option<String>, // Start commit SHA for diff
+    pub position_head_sha: Option<String>, // Head commit SHA for diff
 }
 
 /// Parse ISO 8601 timestamp to milliseconds, returning None on failure.
@@ -113,6 +125,20 @@ pub fn transform_discussion(
     }
 }
 
+/// Transform a GitLab discussion for MR context.
+/// Convenience wrapper that uses NoteableRef::MergeRequest internally.
+pub fn transform_mr_discussion(
+    gitlab_discussion: &GitLabDiscussion,
+    local_project_id: i64,
+    local_mr_id: i64,
+) -> NormalizedDiscussion {
+    transform_discussion(
+        gitlab_discussion,
+        local_project_id,
+        NoteableRef::MergeRequest(local_mr_id),
+    )
+}
+
 /// Transform notes from a GitLab discussion into normalized schema.
 pub fn transform_notes(
     gitlab_discussion: &GitLabDiscussion,
@@ -134,6 +160,20 @@ fn transform_single_note(
     position: i32,
     now: i64,
 ) -> NormalizedNote {
+    // Extract DiffNote position fields if present
+    let (
+        position_old_path,
+        position_new_path,
+        position_old_line,
+        position_new_line,
+        position_type,
+        position_line_range_start,
+        position_line_range_end,
+        position_base_sha,
+        position_start_sha,
+        position_head_sha,
+    ) = extract_position_fields(&note.position);
+
     NormalizedNote {
         gitlab_id: note.id,
         project_id: local_project_id,
@@ -152,9 +192,138 @@ fn transform_single_note(
             .resolved_at
             .as_ref()
             .and_then(|ts| parse_timestamp_opt(ts)),
+        position_old_path,
+        position_new_path,
+        position_old_line,
+        position_new_line,
+        position_type,
+        position_line_range_start,
+        position_line_range_end,
+        position_base_sha,
+        position_start_sha,
+        position_head_sha,
     }
 }
 
+/// Extract DiffNote position fields from GitLabNotePosition.
+/// Returns tuple of all position fields (all None if position is None).
+#[allow(clippy::type_complexity)]
+fn extract_position_fields(
+    position: &Option<crate::gitlab::types::GitLabNotePosition>,
+) -> (
+    Option<String>,
+    Option<String>,
+    Option<i32>,
+    Option<i32>,
+    Option<String>,
+    Option<i32>,
+    Option<i32>,
+    Option<String>,
+    Option<String>,
+    Option<String>,
+) {
+    match position {
+        Some(pos) => {
+            let line_range_start = pos.line_range.as_ref().and_then(|lr| lr.start_line());
+            let line_range_end = pos.line_range.as_ref().and_then(|lr| lr.end_line());
+
+            (
+                pos.old_path.clone(),
+                pos.new_path.clone(),
+                pos.old_line,
+                pos.new_line,
+                pos.position_type.clone(),
+                line_range_start,
+                line_range_end,
+                pos.base_sha.clone(),
+                pos.start_sha.clone(),
+                pos.head_sha.clone(),
+            )
+        }
+        None => (None, None, None, None, None, None, None, None, None, None),
+    }
+}
+
+/// Parse ISO 8601 timestamp to milliseconds with strict error handling.
+/// Returns Err with the invalid timestamp in the error message.
+fn parse_timestamp_strict(ts: &str) -> Result<i64, String> {
+    DateTime::parse_from_rfc3339(ts)
+        .map(|dt| dt.timestamp_millis())
+        .map_err(|_| format!("Invalid timestamp: {}", ts))
+}
+
+/// Transform notes from a GitLab discussion with strict timestamp parsing.
+/// Returns Err if any timestamp is invalid - no silent fallback to 0.
+pub fn transform_notes_with_diff_position(
+    gitlab_discussion: &GitLabDiscussion,
+    local_project_id: i64,
+) -> Result<Vec<NormalizedNote>, String> {
+    let now = now_ms();
+
+    gitlab_discussion
+        .notes
+        .iter()
+        .enumerate()
+        .map(|(idx, note)| transform_single_note_strict(note, local_project_id, idx as i32, now))
+        .collect()
+}
+
+fn transform_single_note_strict(
+    note: &GitLabNote,
+    local_project_id: i64,
+    position: i32,
+    now: i64,
+) -> Result<NormalizedNote, String> {
+    // Parse timestamps with strict error handling
+    let created_at = parse_timestamp_strict(&note.created_at)?;
+    let updated_at = parse_timestamp_strict(&note.updated_at)?;
+    let resolved_at = match &note.resolved_at {
+        Some(ts) => Some(parse_timestamp_strict(ts)?),
+        None => None,
+    };
+
+    // Extract DiffNote position fields if present
+    let (
+        position_old_path,
+        position_new_path,
+        position_old_line,
+        position_new_line,
+        position_type,
+        position_line_range_start,
+        position_line_range_end,
+        position_base_sha,
+        position_start_sha,
+        position_head_sha,
+    ) = extract_position_fields(&note.position);
+
+    Ok(NormalizedNote {
+        gitlab_id: note.id,
+        project_id: local_project_id,
+        note_type: note.note_type.clone(),
+        is_system: note.system,
+        author_username: note.author.username.clone(),
+        body: note.body.clone(),
+        created_at,
+        updated_at,
+        last_seen_at: now,
+        position,
+        resolvable: note.resolvable,
+        resolved: note.resolved,
+        resolved_by: note.resolved_by.as_ref().map(|a| a.username.clone()),
+        resolved_at,
+        position_old_path,
+        position_new_path,
+        position_old_line,
+        position_new_line,
+        position_type,
+        position_line_range_start,
+        position_line_range_end,
+        position_base_sha,
+        position_start_sha,
+        position_head_sha,
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;