feat(intake): talk to the rembg sidecar, always naming u2net (#281)

Adds the client that will let the intake path remove backgrounds from submitted photos via the rembg sidecar over HTTP. isRembgConfigured() reports whether REMBG_URL is set (unconfigured is a normal, working state, not a failure), and removeBackground() posts a file to /api/remove and resolves with the PNG bytes it gets back, rejecting on every failure — unconfigured, unreachable, a non-2xx response, or a body that fails the same magic-byte PNG check the upload path already uses.

The one hard rule: every request names model=u2net explicitly and this is never configurable. The sidecar's default model, reached simply by omitting the parameter, is bria-rmbg, which is licensed non-commercial — a licensing problem that a shop cannot silently ship, and one that would produce a perfectly good image with nothing in it to reveal the mistake. The test that posts against a real stub HTTP server and asserts model=u2net appears on the wire is the only thing guarding against that regressing.

Wires REMBG_URL into both docker-compose.qa.yml and docker-compose.prod.yml as an optional variable, right after ANTHROPIC_WORKSPACE_ID, following the existing style in each file's environment block and header comment. It is deliberately left out of envValidation.ts's ALWAYS_REQUIRED — requiring it would make an environment with no sidecar refuse to boot, which is exactly the failure mode this feature is designed to avoid.

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

docker-compose.qa.yml

@@ -63,6 +63,10 @@
 #                     in the notification email (#224). Absent, the email still
 #                     sends and simply carries no shortcuts. Its own value, not
 #                     production's: a link signed with it acts without a login.
+#   QA_REMBG_URL — optional. The background-removal sidecar, e.g.
+#                     http://rembg-syn:7000. Unset turns the feature off rather
+#                     than breaking anything. The sidecar must be on the same
+#                     network as this stack.
 
 services:
   redefined-designs-qa:
@@ -164,6 +168,12 @@ services:
       # would turn the ordinary case into a different error.
       - ANTHROPIC_WORKSPACE_ID=${QA_ANTHROPIC_WORKSPACE_ID:-}
 
+      # Optional. The background-removal sidecar (#281). Unset means the
+      # feature does not exist: no checkbox on the submission page, no control
+      # in the review queue, and the worker skips the step. Empty default so an
+      # unset stack variable cannot fail a deploy.
+      - REMBG_URL=${QA_REMBG_URL:-}
+
       # Signs the regenerate and discard links in the intake notification email
       # (#224). Optional: absent, the notification still sends and simply links
       # to the review queue without shortcuts. Anyone holding a link can act on