fix: deduplicate distractors, replace tautological test, add distractor failure test

documentation/tickets/t00008.md

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+# fix: deduplicate distractors, replace tautological test
+
+## Problem
+
+Two issues in `createGameSession` and its test suite:
+
+1. If `getDistractors` returned the correct answer as one of the distractors, `createGameSession` would include it in the options array without filtering it out. `indexOf` would then find the first occurrence, which might not be the one intended as the correct answer — producing a question where the correct answer appears twice and the stored `correctOptionId` is wrong.
+
+2. The test `"distractors are never the correct answer"` was tautological — it filtered the correct answer out of the options array, then asserted the remaining items were not the correct answer. It was testing that `Array.filter()` works. It could never fail.
+
+## Options considered
+
+### Option A — Filter duplicates after fetching, request extra distractors as buffer ✅
+
+Filter out any distractor that matches the correct answer after fetching. Request 6 distractors instead of 3 to ensure enough remain after deduplication. Take the first 3 valid ones with `slice(0, 3)`.
+
+Chosen because: deduplication at the service layer is the right place — `getDistractors` shouldn't need to know what the correct answer is. Requesting extra provides a buffer against collisions.
+
+### Option B — Fix `getDistractors` to never return the correct answer
+
+Add a NOT filter in the database query.
+
+Not chosen for this ticket — the database query is in `@lila/db` and is a separate concern. The service layer should be defensive regardless of what the model layer returns.
+
+## Solution
+
+- Filter distractors against the correct answer before building options:
+
+  ```ts
+  const uniqueDistractors = distractorTexts.filter((t) => t !== term.targetText);
+  const optionTexts = [term.targetText, ...uniqueDistractors.slice(0, 3)];
+  ```
+
+- Request 6 distractors instead of 3 to account for potential duplicates
+- Replaced tautological test with a test that actually exercises the duplicate case:
+
+  ```ts
+  it("correct answer appears exactly once even if getDistractors returns a duplicate", ...)
+  ```
+
+- Added distractor failure propagation test:
+
+  ```ts
+  it("propagates getDistractors failure", ...)
+  ```
+
+## Files changed
+
+- `apps/api/src/services/gameService.ts` — deduplication logic, distractor count increased to 6
+- `apps/api/src/services/gameService.test.ts` — tautological test replaced, failure test added
+
+## Commit
+
+`fix: deduplicate distractors, replace tautological test, add distractor failure test`