fix(rmi-backend): drop passlib — route backup codes through direct bcrypt

The reported symptom (AttributeError: module 'bcrypt' has no attribute
'__about__') was misleading. passlib 1.7.4 (the latest released version
— the project is unmaintained) actually traps that AttributeError with
a bare `except:` and continues. The real failure occurs later in
`passlib.handlers.bcrypt._finalize_backend_mixin`:

  detect_wrap_bug -> verify(secret, bug_hash)
    where secret = (b"0123456789"*26)[:255]   # 255-byte test secret

bcrypt 4.0+ removed silent 72-byte truncation and now raises
`ValueError: password cannot be longer than 72 bytes`, breaking the
backend probe and crashing every `pwd_context.hash()/verify()` call.

Three options were considered:

  A. Upgrade passlib — impossible, 1.7.4 is the latest PyPI release.
  B. Pin bcrypt<4.0  — conflicts with chromadb 1.1.1 (>=4.0.1 required),
     even though chromadb's bcrypt usage is optional and still works
     at runtime in practice.
  C. Shim bcrypt.__about__ — doesn't fix the actual truncation error.

Chosen: refactor app/auth.py to use bcrypt directly for backup-code
hashing too (the main `hash_password`/`verify_password` already did).
Drops the passlib dependency entirely, restoring bcrypt 5.x for chromadb
compatibility and eliminating the unmaintained passlib pin.

Verified:
  * hash_password / verify_password round-trip OK
  * _hash_backup_code / _verify_backup_code round-trip OK on
    freshly generated 8-digit backup codes
  * `pytest tests/unit` -> 717 passed, 32 warnings, 60 subtests passed

requirements.txt

@@ -6,7 +6,6 @@ pydantic>=2.9.0
 python-telegram-bot>=21.0
 supabase>=2.0.0
 python-jose[cryptography]>=3.3.0
-passlib[bcrypt]>=1.7.4
 python-multipart>=0.0.17
 aiohttp>=3.11.0
 feedparser>=6.0.11