v0.7 CAD lane: switch processing flows to the server-owned proxy path

[SheetMetalConnect]

Linear: ERY-12
Depends on: #890
Umbrella context: #561
Planning source: work/ERY-346-v0.7-execution-breakdown.md

Objective

• Complete the v0.7 CAD feature lane after proxy deployment and secret rotation are in place.

Scope

• Route CAD upload/processing flows through the server-owned proxy path.
• Preserve geometry, thumbnail, and metrics outputs expected from the CAD pipeline.
• Capture one end-to-end verification artifact showing the proxy path works.

Acceptance criteria

• Client CAD flows use the server-owned path rather than direct backend-key calls.
• Geometry, thumbnails, or equivalent CAD outputs still land successfully.
• Verification artifact is attached or linked in the issue.