Backup: zip-compress output (.pgn.zip) instead of plain PGN (#91)

[jozef2svrcek]

Closes #91.

chess-db backup (and the GUI's Backup action) now writes a timestamped .pgn.zip — a single Deflate-compressed .pgn entry — instead of an uncompressed .pgn.

Why zip (and why not zstd)

Measured both on a representative ~13 MB multi-game PGN (high-entropy movetext, so a pessimistic floor):

format	ratio	vs zip -9
zip -9 (DEFLATE)	2.87×	—
zstd -3 (default)	2.72×	6% worse
zstd -19	3.54×	20% smaller

zstd only wins if cranked to a high level, and even then by ~20% — while .zip is double-clickable on every OS. For a backup artifact, native openability beats a modest size edge, so we default to zip. Both zip and zstd were already dependencies (zip is used on the import side).

Notes

• The entry round-trips: a .pgn.zip backup re-imports through the importer's existing zip reader (a single .pgn entry inside).
• Games are streamed straight into the archive rather than concatenated into one big in-memory String first (incidental memory win; the row load itself is still Importer: stream large PGN imports instead of loading the whole file in memory #95's concern).
• Added a round-trip unit test (backup → open the zip → both games present); cargo test -p chess-db (14 passed) and cargo clippy -- -D warnings clean; client builds.

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