docs(m3): close create contract gaps (soft delete, publish poll, prompts, image volume)

- R1 soft delete: add deleted_at to slabs; exclude deleted rows from GETs
- R2 publish: terminal poll returns woo_product_id then the working row is
  discarded; the app never deletes from WooCommerce (reworks PUBLISH-25)
- R3 prompts: SQLite is the point of record; file-based prompts superseded,
  compose prompt mount removed
- R4 images: photo bytes are SQLite BLOBs; /images is a health probe + staging

Details recorded in .audit/m3-build-plan.md.

deploy/docker-compose.yml

@@ -1,6 +1,6 @@
 # Implementable compose for SlabUploader.
 # Phase 0 deliverable targets: frontend/Dockerfile, backend/Dockerfile,
-# backend/app/prompts/, and deploy/.env.example are not yet in the tree on
+# and deploy/.env.example are not yet in the tree on
 # docs-only main. This file pins their paths so scaffolding does not invent them.
 # From repo root (after those scaffolds exist):
 #   cp deploy/.env.example deploy/.env && chmod 600 deploy/.env
@@ -34,7 +34,6 @@ services:
     volumes:
       - slab_db:/data
       - slab_images:/images
-      - ../backend/app/prompts:/app/prompts:ro
     ports: ["127.0.0.1:${FASTAPI_PORT:-18081}:8000"]
     mem_limit: 2g
     restart: unless-stopped