docs: document the last play date rule property and notification grouping

[enoch85]

Closes #150. Doc updates for the reported drift, confirmed against the upstream commits in a local clone of Maintainerr rather than the report summary. Glossary parity is back at 210/210 keys.

What was added

File	Change	Upstream
docs/Glossary.md	Plex.lastPlayedAt: last play including one stopped early. Reports the same date as Plex.lastViewedAt because Plex moves an item's own last-viewed date on any playback, and it reads that field instead of the watch history	Maintainerr#3508
docs/Glossary.md	Jellyfin.lastPlayedAt, also on Emby: newest play date across all users, episodes walked for shows and seasons, unknown when the date cannot be read for every user	Maintainerr#3505
docs/Glossary.md	Tautulli.lastPlayedAt and Tracearr.lastPlayedAt: newest recorded session, ignoring the watched percentage and the collection's Watched percent override	Maintainerr#3508
docs/Glossary.md	Streamystats.lastPlayedAt: last play by anyone, unavailable for seasons, unknown when item details cannot be read	Maintainerr#3508
docs/Rules.mdx	Watch History Rules block: played versus viewed, which services offer the property, and why the two report the same date on Plex	Maintainerr#3505, Maintainerr#3508
docs/Notifications.md	Removals are collected per collection during a collection handling run and sent as one message at the end, rule runs report theirs during the run, rate-limited sends are retried up to a minute, Discord messages are truncated	Maintainerr#3506

Already covered by prior PRs

Drift item	Where
Emby.lastPlayedAt	Emby mirrors Jellyfin's property set, already stated in the Jellyfin Glossary note, so the property is noted in the Jellyfin entry instead of a separate Emby section
Watch-history lookups staying unknown on failure	#140, the new entries only point at the same rule
Migrations, contracts, controllers	none in this drift