chore(streak): formalise deprecation status per SB-9

ls src/app/api/streak returns No such file or directory. No web
frontend code paths reference /api/streak/...; the only streak
references in the API tree are derived fields inside
/api/insights/targets (streakDays, streakHighlight).

Decision: nothing to deprecate. The route family either never existed
or was removed before the audit. iOS can safely treat /api/streak/*
as 404. Document filed so a future audit does not re-open the
question.

.planning/sb-9-streak-deprecation.md

@@ -0,0 +1,30 @@
+# SB-9 — Streak endpoint deprecation decision
+
+**Owner:** W-IOS-COORD (v1.4.41)
+**Decision:** **already retired — nothing to deprecate.**
+
+## Investigation
+
+iOS v1.4.40 PB1 noted it stops calling `/api/streak/*`. The backlog
+flagged that the server might want to either keep responding 200 for
+the web app or fully retire the routes.
+
+`ls src/app/api/streak` returns `No such file or directory`. There
+are no `route.ts` files under that path. No web frontend code paths
+fetch `/api/streak/...`. The only `streak` references in the API are
+internal computation inside `src/app/api/insights/targets/route.ts`
+(`streakDays`, `streakHighlight`) which is a derived field on an
+unrelated payload, not a standalone route.
+
+## Conclusion
+
+There is no `/api/streak/*` surface to deprecate. The iOS team's PB1
+note refers to a route family that either never existed on this server
+or was removed before the audit. No code action required.
+
+## Action
+
+- No code change.
+- Documented here so a future audit doesn't re-open the question.
+- iOS team can safely treat `/api/streak/*` as 404 — the web app does
+  not depend on the route either.