feat: mark video as watched after a certain duration

[goestav]

This pull request marks videos as watched after watching them for a given amount of time and also indirectly fixes a bug that prevents YouTube/Invidious links from updating when seeking backwards for the video.

Closes #3794

Todo

• Mark as watched after watching for at least 30 seconds or fully watching
• Add preference for this feature
  • Default to false to avoid confusion for existing users
  • Store value in browser's local storage
• Remove server side logic that marks a video as watched when navigating to a video

[Copilot]

Pull Request Overview

This PR introduces a feature to automatically mark videos as watched after the user has watched them for at least 30 seconds (or fully for videos shorter than 30 seconds). The feature is opt-in via a new user preference and uses client-side tracking to determine when to mark videos as watched. It also removes the previous server-side logic that marked videos as watched upon navigation to a video page, and fixes a bug preventing YouTube/Invidious link updates when seeking backwards.

Key changes:

• Added mark_watched_after_duration preference with default value of false to avoid confusion for existing users
• Implemented client-side tracking in player.js to monitor watch time and trigger mark-as-watched after thresholds are met
• Removed automatic server-side marking of videos as watched on page load

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/invidious/config.cr	Added mark_watched_after_duration property to default preferences config
src/invidious/user/preferences.cr	Added mark_watched_after_duration property to user preferences struct
src/invidious/videos/video_preferences.cr	Added processing and handling of mark_watched_after_duration parameter in video preferences
src/invidious/routes/preferences.cr	Added route handling to save the mark_watched_after_duration preference from form submission
src/invidious/routes/watch.cr	Removed server-side logic that automatically marked videos as watched on page navigation
src/invidious/views/user/preferences.ecr	Added checkbox UI for the new preference in the preferences page
src/invidious/views/template.ecr	Added hidden span to expose the preference value to client-side JavaScript (only for logged-in users)
locales/en-US.json	Added English translation for the preference label
assets/js/player.js	Implemented client-side watch time tracking and auto-mark-as-watched logic, also fixed backward seeking bug

Comments suppressed due to low confidence (1)

assets/js/player.js:210

• Avoid automated semicolon insertion (93% of all statements in the enclosing function have an explicit semicolon).

    time_watched += 1

---

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[unixfox]

It also removes the previous server-side logic that marked videos as watched upon navigation to a video page

I don't think that's a good thing because we still want to support non js users as long as possible.

[goestav]

Apologies, that was supposed to be ignored only if this feature is enabled.

- if watched && preferences.watch_history
+ if watched && preferences.watch_history && !preferences.mark_watched_after_duration
  Invidious::Database::Users.mark_watched(user.as(User), id)
end

It should probably be mentioned under the checkbox in the preference page that the feature requires JavaScript though.
What do you think?