fix(database): serialize incident creation per server

[passcod]

Summary

• Two concurrent event pushes against different issues on the same server could each insert a fresh incident row, leaving two open incidents for one server. The per-issue `FOR UPDATE` on the issue row doesn't help — the two events lock different issue rows, so they don't conflict at the issue level, but they then race on the find-or-create in `find_or_open_incident`.
• Take a `SELECT ... FOR UPDATE` on the root server's row at the top of `find_or_open_incident` so the find-or-create pair serializes per-server.
• Promote the existing `incidents_open_by_server` partial index on `(server_id) WHERE closed_at IS NULL` to UNIQUE. Belt-and-braces backstop at the DB layer: even if the lock is ever bypassed, the second concurrent insert fails outright.

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