ci(ci): add SRS_ADMIN_KEY env var to spaced-repetition workflow

.github/workflows/spaced-repetition.yml

@@ -1,10 +1,11 @@
 name: Spaced Repetition
 
 # Opens (or refreshes) today's `Spaced Repetition — <Month D, YYYY>` issue from
-# the problem issues closed 3 and 7 days ago. No stored state: the script
-# recomputes both windows from the tracker's close dates on every run, keyed by
-# the ET date in the title, so a re-run rewrites one body instead of stacking
-# duplicates and a missed day simply never appears.
+# whatever the SRS ladder has due (`POST /admin/due`, the same rows the 8 AM
+# digest mails — so the two cannot disagree). The script stores nothing and
+# schedules nothing: it asks D1 what is standing today, keyed by the ET date in
+# the title, so a re-run rewrites one body instead of stacking duplicates.
+# Sunday is the rest day and the ladder never books it, so nothing is written.
 
 on:
   schedule:
@@ -40,8 +41,14 @@ jobs:
       - uses: oven-sh/setup-bun@v2
 
       # No install step: the script only uses Bun builtins + fetch.
+      #
+      # SRS_ADMIN_KEY reads today's due list from the Worker. Unset is fatal
+      # here, unlike in close-solved: without D1 there is no schedule to render,
+      # and inventing one from issue close dates is exactly the second source of
+      # truth this replaced.
       - run: bun apps/cli/spaced-repetition.ts
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           DRY_RUN: ${{ inputs.dry_run }}
           SRS_TODAY: ${{ inputs.date }}
+          SRS_ADMIN_KEY: ${{ secrets.SRS_ADMIN_KEY }}