[Backport 18.8]Fix PG::UniqueViolation in project_daily_statistics sync trigger

What does this MR do and why?

Related: #592642 (closed)

After the project_daily_statistics partitioning table swap in !211916 (merged), the sync trigger table_sync_function_c237afdf68() copies writes from the new partitioned table to project_daily_statistics_archived (the original table). The trigger's INSERT branch performs a plain INSERT without conflict handling.

When a row for the same (project_id, date) already exists in the archived table, the trigger raises PG::UniqueViolation, which causes an HTTP 500 on POST /git-upload-pack. This breaks all CI pipeline git clone operations after the first fetch per day per project.

This MR adds ON CONFLICT DO NOTHING to the trigger's INSERT branch, so duplicate rows in the archived table are silently skipped.

Note:

This issue only affects 18.8.x. In 18.9, the archived table and trigger were removed entirely in !219031 (merged).

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