fix: add bcrypt version constraint for passlib compatibility

bcrypt 5.0.0 is incompatible with passlib 1.7.x - passlib tries to
access bcrypt.__about__.__version__ which was removed in bcrypt 5.x.

Constrain bcrypt to >=4.0,<5.0 to maintain compatibility.

src/backend/requirements.txt

@@ -6,6 +6,7 @@ pydantic-settings>=2.1.0
 email-validator>=2.0.0
 python-jose[cryptography]>=3.3.0
 passlib[bcrypt]>=1.7.4
+bcrypt>=4.0,<5.0  # Required for passlib compatibility
 crewai>=0.1.0
 anthropic>=0.18.0
 httpx>=0.26.0