build(intake): give the containers an INTAKE_ACTION_SECRET (#224)

A gap in the plan rather than in the code: nothing wired the secret into either stack, so the feature would have shipped with its signed links permanently disabled and nothing saying why. Both compose files now interpolate it, with `:-` so an unset variable stays empty rather than failing the deploy.

QA takes QA_INTAKE_ACTION_SECRET, its own value rather than production's, for the same reason as every other QA_ prefixed credential — and more sharply here, because a link signed with it acts on a draft without a login.

Rotating the secret revokes every outstanding link, which is the intended answer to one leaking.

The cutover doc counted fourteen interpolated names and now counts fifteen. That document says it is checked against the file rather than from memory, so it was: fifteen in the compose file, the same fifteen listed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -92,6 +92,9 @@
 #                         failing, so an empty value is a working configuration.
 #   ANTHROPIC_API_KEY     Optional. Drafts a listing from a submitted photo
 #                         (#223). Unset means submissions still arrive and wait
+#   INTAKE_ACTION_SECRET  Optional. Signs the regenerate and discard links in the
+#                         intake notification email (#224). Absent, the email
+#                         still sends and carries no shortcuts.
 #                         undrafted, which is a working configuration for the
 #                         same reason USPS is. The one credential here that
 #                         spends money per call, and on a path anybody holding
@@ -225,6 +228,12 @@ services:
       # one, and #227 is the submission ceiling that bounds the volume rather
       # than the bill.
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY:-}
+
+      # Signs the regenerate and discard links in the intake notification email
+      # (#224). Optional: absent, the notification still sends and links to the
+      # review queue without shortcuts. Rotating it revokes every outstanding
+      # link, which is how a leaked one is dealt with.
+      - INTAKE_ACTION_SECRET=${INTAKE_ACTION_SECRET:-}
     volumes:
       # Production's own uploads directory. QA writes to
       # /volume1/configs/redefined-designs-qa/uploads; sharing this one would