feat: deduplicate markLessonComplete with 500ms guard window

[mrteeednut007-dotcom]

Deduplicate markLessonComplete with 500 ms guard window

Problem

In the video player, lesson completion fires from two sources — video end and manual skip. When
both triggers fire within milliseconds of each other (e.g. a user skips a short lesson right as
it ends), markLessonComplete was called twice in rapid succession. This caused:

• Duplicate completion records written to the server
• overallProgress double-incrementing
• Potential false course-completion events

Solution

Added a module-level completionInProgress Set that acts as a per-lesson deduplication guard:

• On the first call for a given courseId:lessonId key, the optimistic state update fires
  normally and the key is added to the set
• Any subsequent call within 500 ms hits the guard and returns early — no state mutation, no
  API call
• After 500 ms the key is removed, so a genuine retry or re-completion works as expected
• Guard is scoped to courseId:lessonId so different lessons and different courses are never
  blocked by each other

Changes

• src/store/courseProgressStore.ts — added completionInProgress Set, _completionTimers Map, and
  guard logic at the top of markLessonComplete
• src/tests/store/courseProgressStore.test.ts — 6 new tests covering all acceptance
  criteria

Tests

All 11 tests pass (npm test):

┌──────────────────────────────────────┬────────┐
│ Test │ Covers │
├────────────────────────────────────────────────┼─────────────────────────────┤
│ Second call within 500 ms is ignored │ AC1 — single state mutation │
├────────────────────────────────────────────────┼─────────────────────────────┤
│ Two rapid calls → 1 mutation (state assertion) │ AC1 — no double update │
├────────────────────────────────────────────────┼─────────────────────────────┤
│ Call after 500 ms proceeds normally │ AC2 — third call works │
├────────────────────────────────────────────────┼─────────────────────────────┤
│ overallProgress increments exactly once │ AC3 — no double increment │
├────────────────────────────────────────────────┼─────────────────────────────┤
│ Guard is per lessonId │ Isolation │
├────────────────────────────────────────────────┼─────────────────────────────┤
│ Guard is per courseId │ Isolation │
└────────────────────────────────────────────────┴─────────────────────────────┘

close #628

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