Backend major dependency upgrades (bcrypt, redis, mypy)

[codethor0]

Summary

Track intentional major backend dependency upgrades deferred during the post-v0.1.1 Dependabot cleanup.

Deferred Dependabot PRs

Package	From	To	PR
bcrypt	4.2.1	5.0.0	#23
redis	5.2.1	8.0.1	#25
mypy	1.13.0	2.2.0	#26

Why deferred

These are major semantic-version jumps requiring dedicated migration review, not batch closure to reduce branch count.

Required before merge

• Review changelogs and breaking changes
• Run full backend pytest, mypy, bandit, pip-audit
• Run live E2E validation against Docker stack
• Confirm password hashing behavior for bcrypt 5.x
• Confirm redis client API compatibility with existing async usage
• Confirm mypy strict mode passes with new release

Notes

Routine minor/patch Python updates remain in grouped PR #21.