Bulk copy pushed turns

[upsetbit]

Closes #106.

Summary

• replace per-turn Postgres inserts in replaceTurns with tx.CopyFrom
• keep the existing delete-then-replace transaction semantics
• extend the server e2e Push test to cover default turn kind and tool evidence metadata

Local validation

• PROSA_TEST_PG_URL=postgres://prosa:prosa@localhost:55432/prosa?sslmode=disable go test ./internal/server/handlers -run 'TestSessionsConnectEndToEnd|TestSessionsPush' -count=1 -v\n- PROSA_TEST_PG_URL=postgres://prosa:prosa@localhost:55432/prosa?sslmode=disable go test -race -count=1 ./internal/server/...\n- devbox run -- just lint\n- PROSA_TEST_PG_URL=postgres://prosa:prosa@localhost:55432/prosa?sslmode=disable just test-race

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