feat(scripts): fold model downloads into update.sh, skip if already present

Today's embedding-server crash-loop (missing nomic-embed-text GGUF) was a
manual step nobody ran. update.sh now runs both downloader profiles
itself, every time, before bringing services up -- no separate command to
remember.

- docker-compose.yml: downloader/downloader-embedding commands gain a
  `test -f ... && skip || curl ...` guard, so re-running update.sh never
  re-downloads an existing model file.
- scripts/update.sh: runs both profiles after image pull/build, before
  service recreation.
- scripts/download-model.sh removed -- folded in, redundant standalone
  script.
- README.md / docs/memory-knowledgebase.md updated accordingly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018WHfjWrSEcGhCoeu6dQfDa

docs/memory-knowledgebase.md

@@ -32,6 +32,8 @@ New pieces:
 
 ### First-time setup
 
+`./scripts/update.sh` fetches the embedding model automatically (skips it if already downloaded). To do it by hand instead:
+
 ```bash
 docker compose --profile tools run --rm downloader-embedding   # fetch the embedding model
 docker compose up -d embedding-server pgvector-db litellm-pgvector