fix: credential lifecycle safety -- update_item, duplicate prevention, tests

CRITICAL FIX: The --enable-2fa flow was creating duplicate BW items
instead of updating in place, which led to ambiguous item resolution
and data integrity issues. This was a severe failure in core
credential lifecycle operations.

Changes:
- bw_helper.py: Complete rewrite with safety guarantees
  - update_item(): modifies existing item in place by ID, preserves
    all fields not being updated
  - create_item(): refuses to create duplicates (raises if item exists)
  - get_item_id(): resolves name to ID, raises on ambiguous matches
  - get_item(): returns full item JSON
  - NO delete_item method exists by design -- credential deletion
    is a manual operation only
- provision-agent.py: --enable-2fa now uses update_item() instead
  of create_item() to add TOTP to existing credentials
- Dockerfile: non-root user with correct BW state directory ownership
- docker-compose.yml: bind mount for BW state (proper permissions)
- test_bw_helper.py: 10 tests covering full lifecycle
  (create, read, duplicate rejection, update password, update TOTP,
  field preservation, no-delete verification)
  Tests 1-4 verified passing against live Vaultwarden instance.
- requirements.txt: added pytest

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

requirements.txt

@@ -4,3 +4,4 @@ pyotp==2.9.0
 qrcode==7.4.2
 Pillow==10.4.0
 pyzbar==0.1.9
+pytest==8.3.2