Fix #169: keep TV episodes contiguous when cache space is tight

[Brandon-Haney]

Fixes #169.

When cache space is tight, the next OnDeck episode (e.g. S04E03) could be skipped while later episodes of the same show (E04, E05) got cached anyway — leaving a viewing gap. Two layers were contributing.

What was happening

1. Ordering was destroyed before the space-fit step. _gather_media_to_cache collected paths into a set() and then did list(modified_paths_set) to hand off to _apply_cache_limit. Python set has no defined iteration order, so OnDeck's "current + next N episodes" sequence arrived at the fitter in arbitrary hash-bucket order.

2. _apply_cache_limit was a greedy first-fit pack. If an episode didn't fit, the loop just skipped it and kept going. So a 15 GB 4K E03 with 10 GB free would be skipped, then 2 GB E04 + 2 GB E05 would both fit and get cached.

What this PR does

1. modified_paths_set (a set) becomes modified_paths: Dict[str, None] used as an ordered set — Python dicts preserve insertion order, so OnDeck fetch order survives all the way through. Final priority order: pinned → OnDeck → OnDeck siblings → watchlist.

2. _apply_cache_limit now tracks shows_at_capacity. Once an episode of show X doesn't fit, every later file mapped to show X via media_info_map is skipped, even if it would individually fit. Movies and other shows still pack independently into the remaining space, so one oversized 4K episode doesn't starve everything else.

3. The space-limit warning now appends (N skipped to keep show episodes contiguous) when gap-skips happen, so the behavior is visible in logs.

Test plan

• Configure a cache quota or min-free-space so total OnDeck size exceeds available space
• Run the cache and confirm: either all of the in-order next episodes for that show fit, or none do — no gaps mid-sequence
• Confirm other shows and movies still cache normally into remaining space
• Check logs for the (N skipped to keep show episodes contiguous) note when applicable

New TestNoGapShowOrdering class in tests/test_plexcache_quota.py covers:

• Oversized first episode → later episodes of same show skipped
• Mid-sequence break (E03 fits, E04 oversized → E05 skipped)
• Cross-show independence (ShowA at capacity doesn't starve ShowB or movies)
• No-metadata fallback (movies/siblings without show keys keep first-fit behavior)
• Happy path (all fit → all cached, no skips)