fix(ingestion): label upsert reliability, init idempotency, and sync health

Label upsert (issues + merge_requests): Replace INSERT ... ON CONFLICT DO
UPDATE RETURNING with INSERT OR IGNORE + SELECT. The prior RETURNING-based
approach relied on last_insert_rowid() matching the returned id, which is
not guaranteed when ON CONFLICT triggers an update (SQLite may return 0).
The new two-step approach is unambiguous and correctly tracks created_count.

Init: Add ON CONFLICT(gitlab_project_id) DO UPDATE to the project insert
so re-running `lore init` updates path/branch/url instead of failing with
a unique constraint violation.

MR discussions sync: Reset discussions_sync_attempts to 0 when clearing a
sync health error, so previously-failed MRs get a fresh retry budget after
successful sync.

Count: format_number now handles negative numbers correctly by extracting
the sign before inserting thousand-separators.

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

src/ingestion/mr_discussions.rs

@@ -593,6 +593,7 @@ fn clear_sync_health_error(conn: &Connection, local_mr_id: i64) -> Result<()> {
     conn.execute(
         "UPDATE merge_requests SET
             discussions_sync_last_attempt_at = ?,
+            discussions_sync_attempts = 0,
             discussions_sync_last_error = NULL
          WHERE id = ?",
         params![now_ms(), local_mr_id],