feat(settings): S1 AES-GCM crypto + Settings API/UI (masked write-only secrets)

- backend/crypto.py: AES-GCM (cryptography.AESGCM); key=base64(SLAB_AES_KEY)
  must be exactly 32 bytes; value_enc=base64(nonce(12)||ct); DecryptError on tamper
- backend/settings.py: get_settings (secrets->{configured:bool}, woo_base_url env
  read-only, flags default false), update_settings (rejects woo_base_url SETTINGS-85,
  encrypts secrets, partial plain writes)
- backend/app.py: wire GET/PUT /api/v1/settings
- frontend/src/create/settings/Settings.svelte: masked form + partial PUT
- tests: crypto roundtrip/tamper, masked GET, encrypted PUT, reject woo_base_url,
  flags default, decrypt-fail->configured:false (TDD)

Verified live: secrets round-trip configured:true with valid key; woo_base_url write
rejected 400; no secret leak in GET. Gates: check_requirements + check_coverage exit 0;
pytest 13 passed. Deploy+build on ubudual5060 (never Windows).

backend/requirements.txt

@@ -3,4 +3,5 @@ fastapi==0.115.2
 uvicorn[standard]==0.30.6
 pytest==8.3.3
 httpx==0.27.2
+cryptography==43.0.1
 # stdlib only for sqlite3, logging, json, contextvars, threading