Replace passlib with direct bcrypt calls, drop passlib dependency

passlib is unmaintained and incompatible with bcrypt 5.0.0 due to its
internal wrap-bug detection test exceeding bcrypt's 72-byte password limit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/requirements.txt

@@ -5,6 +5,5 @@ pymysql==1.2.0
 cryptography==48.0.0
 pydantic==2.13.4
 python-jose[cryptography]==3.5.0
-passlib[bcrypt]==1.7.4
 bcrypt==5.0.0
 httpx==0.28.1