Pin bcrypt<4.1 to fix passlib 1.7.4 compatibility

passlib 1.7.4 (last release, unmaintained since ~2020) detects its bcrypt
backend via bcrypt.__about__.__version__, which bcrypt>=4.1 removed. Every
hash_password/verify_password call was raising 'password cannot be longer
than 72 bytes' -- a misleading downstream symptom of the real
AttributeError: module 'bcrypt' has no attribute '__about__'.

Without this pin, pip install -r requirements.txt silently grabs whatever
the latest bcrypt is and breaks register/login outright -- confirmed this
broke 3 separate pieces of work this session (manual register curl tests,
and now a real Godot smoke test) purely from environment drift, not from
any actual code bug. Verified the fix works from a genuinely clean venv +
pip install, not just patching the already-installed shared venv.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/requirements.txt

@@ -9,6 +9,14 @@ pydantic==2.10.3
 pydantic-settings==2.7.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
+# passlib 1.7.4 (last release, unmaintained since ~2020) detects its bcrypt
+# backend via bcrypt.__about__.__version__, which bcrypt>=4.1 removed --
+# hashing/verifying a password then raises "password cannot be longer than
+# 72 bytes" (a misleading downstream symptom of the real AttributeError:
+# module 'bcrypt' has no attribute '__about__') on every call. Without this
+# pin, `pip install -r requirements.txt` silently grabs whatever the latest
+# bcrypt is and breaks register/login outright. Pin below 4.1 explicitly.
+bcrypt==4.0.1
 python-multipart==0.0.20
 email-validator==2.2.0
 httpx==0.28.1