Pass captured transaction to ExecuteOnConnection callback instead of re-reading field

[Copilot]

Callers like WriteImageFile and PostUpdateProcessing were re-reading this.transaction inside their lambdas without holding transactionLock. When no active transaction exists, the lambda falls through to the unlocked path at line 90 — a concurrent BeginTransaction between the check and the callback could cause the lambda to execute against a mismatched connection/transaction pair.

Changes

• ExecuteOnConnection signature: Func<SqliteConnection, T> → Func<SqliteConnection, SqliteTransaction?, T>
• Inside lock: passes lock-captured currentTransaction directly to callback — func(currentTransaction.Connection, currentTransaction)
• No-transaction path: passes null — func(connection, null)
• Transactional callers (WriteImageFile, PostUpdateProcessing): updated to use the provided tx parameter instead of this.transaction
• Non-transactional callers: use (connection, _) discarding the parameter

// Before — re-reads this.transaction outside the lock
ExecuteOnConnection(connection => connection.Execute(sql, param, this.transaction));

// After — uses the lock-captured reference passed by ExecuteOnConnection
ExecuteOnConnection((connection, tx) => connection.Execute(sql, param, tx));

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