feat: Add row_changes_cap_per_file parameter to datasets comparer

[jcpitre]

Adds an optional row_changes_cap_per_file parameter to the gtfs_datasets_comparer HTTP function, controlling the maximum number of row-level change entries included per file in the generated changelog. Defaults to 10000 to prevent excessively large outputs on big feeds. Set to 0 to omit row-level detail entirely, or null for no cap.

Invalid (non-integer) values are rejected early with a clear error message rather than bubbling up as a generic exception.

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