fix(cycle): answer a raced feed rotation with 409 instead of 500

Two rotations at once both revoked the same live feed, and the second
successor then hit the one-live-feed index with an unhandled unique
violation. The losing rotation now gets the mint path's 409
cycle.feed.exists, and the winner's link stands.

docs/api/openapi.yaml

@@ -958,7 +958,8 @@ paths:
         - Cycle
       summary: Replace the calendar subscription's link (#1350)
       description: "One transaction: the live link is revoked and a new one minted with the same options. Returns the new URL
-        EXACTLY ONCE. Calendars using the old link stop updating."
+        EXACTLY ONCE. Calendars using the old link stop updating. Of two rotations racing, the one that loses answers
+        409 `cycle.feed.exists` and the winner's link stands."
       responses:
         "201": *a13
         "401": *a14
@@ -969,6 +970,12 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
+        "409":
+          description: Another rotation replaced the link at the same moment (`cycle.feed.exists`).
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
         "422": *a4
         "429": *a5
   /api/measurement-categories: