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/merge_request.rs

@@ -0,0 +1,155 @@
+//! Merge request transformer: converts GitLabMergeRequest to local schema.
+
+use chrono::DateTime;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use crate::gitlab::types::GitLabMergeRequest;
+
+/// Get current time in milliseconds since Unix epoch.
+fn now_ms() -> i64 {
+    SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .expect("Time went backwards")
+        .as_millis() as i64
+}
+
+/// Parse ISO 8601 timestamp to milliseconds since Unix epoch.
+fn iso_to_ms(ts: &str) -> Result<i64, String> {
+    DateTime::parse_from_rfc3339(ts)
+        .map(|dt| dt.timestamp_millis())
+        .map_err(|e| format!("Failed to parse timestamp '{}': {}", ts, e))
+}
+
+/// Parse optional ISO 8601 timestamp to optional milliseconds since Unix epoch.
+fn iso_to_ms_opt(ts: &Option<String>) -> Result<Option<i64>, String> {
+    match ts {
+        Some(s) => iso_to_ms(s).map(Some),
+        None => Ok(None),
+    }
+}
+
+/// Local schema representation of a merge request row.
+#[derive(Debug, Clone)]
+pub struct NormalizedMergeRequest {
+    pub gitlab_id: i64,
+    pub project_id: i64,
+    pub iid: i64,
+    pub title: String,
+    pub description: Option<String>,
+    pub state: String,
+    pub draft: bool,
+    pub author_username: String,
+    pub source_branch: String,
+    pub target_branch: String,
+    pub head_sha: Option<String>,
+    pub references_short: Option<String>,
+    pub references_full: Option<String>,
+    pub detailed_merge_status: Option<String>,
+    pub merge_user_username: Option<String>,
+    pub created_at: i64,        // ms epoch UTC
+    pub updated_at: i64,        // ms epoch UTC
+    pub merged_at: Option<i64>, // ms epoch UTC
+    pub closed_at: Option<i64>, // ms epoch UTC
+    pub last_seen_at: i64,      // ms epoch UTC
+    pub web_url: String,
+}
+
+/// Merge request bundled with extracted metadata.
+#[derive(Debug, Clone)]
+pub struct MergeRequestWithMetadata {
+    pub merge_request: NormalizedMergeRequest,
+    pub label_names: Vec<String>,
+    pub assignee_usernames: Vec<String>,
+    pub reviewer_usernames: Vec<String>,
+}
+
+/// Transform a GitLab merge request into local schema format.
+///
+/// # Arguments
+/// * `gitlab_mr` - The GitLab MR API response
+/// * `local_project_id` - The local database project ID (not GitLab's project_id)
+///
+/// # Returns
+/// * `Ok(MergeRequestWithMetadata)` - Transformed MR with extracted metadata
+/// * `Err(String)` - Error message if transformation fails (e.g., invalid timestamps)
+pub fn transform_merge_request(
+    gitlab_mr: &GitLabMergeRequest,
+    local_project_id: i64,
+) -> Result<MergeRequestWithMetadata, String> {
+    // Parse required timestamps
+    let created_at = iso_to_ms(&gitlab_mr.created_at)?;
+    let updated_at = iso_to_ms(&gitlab_mr.updated_at)?;
+
+    // Parse optional timestamps
+    let merged_at = iso_to_ms_opt(&gitlab_mr.merged_at)?;
+    let closed_at = iso_to_ms_opt(&gitlab_mr.closed_at)?;
+
+    // Draft: prefer draft, fallback to work_in_progress
+    let is_draft = gitlab_mr.draft || gitlab_mr.work_in_progress;
+
+    // Merge status: prefer detailed_merge_status over legacy
+    let detailed_merge_status = gitlab_mr
+        .detailed_merge_status
+        .clone()
+        .or_else(|| gitlab_mr.merge_status_legacy.clone());
+
+    // Merge user: prefer merge_user over merged_by
+    let merge_user_username = gitlab_mr
+        .merge_user
+        .as_ref()
+        .map(|u| u.username.clone())
+        .or_else(|| gitlab_mr.merged_by.as_ref().map(|u| u.username.clone()));
+
+    // References extraction
+    let (references_short, references_full) = gitlab_mr
+        .references
+        .as_ref()
+        .map(|r| (Some(r.short.clone()), Some(r.full.clone())))
+        .unwrap_or((None, None));
+
+    // Head SHA
+    let head_sha = gitlab_mr.sha.clone();
+
+    // Extract assignee usernames
+    let assignee_usernames: Vec<String> = gitlab_mr
+        .assignees
+        .iter()
+        .map(|a| a.username.clone())
+        .collect();
+
+    // Extract reviewer usernames
+    let reviewer_usernames: Vec<String> = gitlab_mr
+        .reviewers
+        .iter()
+        .map(|r| r.username.clone())
+        .collect();
+
+    Ok(MergeRequestWithMetadata {
+        merge_request: NormalizedMergeRequest {
+            gitlab_id: gitlab_mr.id,
+            project_id: local_project_id,
+            iid: gitlab_mr.iid,
+            title: gitlab_mr.title.clone(),
+            description: gitlab_mr.description.clone(),
+            state: gitlab_mr.state.clone(),
+            draft: is_draft,
+            author_username: gitlab_mr.author.username.clone(),
+            source_branch: gitlab_mr.source_branch.clone(),
+            target_branch: gitlab_mr.target_branch.clone(),
+            head_sha,
+            references_short,
+            references_full,
+            detailed_merge_status,
+            merge_user_username,
+            created_at,
+            updated_at,
+            merged_at,
+            closed_at,
+            last_seen_at: now_ms(),
+            web_url: gitlab_mr.web_url.clone(),
+        },
+        label_names: gitlab_mr.labels.clone(),
+        assignee_usernames,
+        reviewer_usernames,
+    })
+}