legacy: move legacy AES-CTR key classes into borg.legacy.crypto, refs #9556

[mr-raj12]

Description

Moves the four Borg 1.x AES-CTR key classes into borg.legacy.crypto, refs #9556.

• KeyfileKey, RepoKey, Blake2KeyfileKey, Blake2RepoKey → legacy/crypto/key.py
• LEGACY_KEY_TYPES moved with them
• borg.crypto.key re-exports all four classes and LEGACY_KEY_TYPES for backwards compatibility
• Shared base classes (FlexiKey, AESKeyBase, ID_HMAC_SHA_256, ID_BLAKE2b_256) remain in borg.crypto.key and are imported by the legacy module (no duplication)

No logic changes. The legacy import in borg.crypto.key is placed after FlexiKey is fully defined so the partial module already has all needed names during the circular load.

Refs #9556

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.34%. Comparing base (62d8762) to head (0cf9322).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9600   +/-   ##
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  Coverage   83.34%   83.34%           
=======================================
  Files          88       89    +1     
  Lines       15541    15546    +5     
  Branches     2342     2342           
=======================================
+ Hits        12952    12957    +5     
  Misses       1835     1835           
  Partials      754      754           

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[ThomasWaldmann]

LGTM! This was about the repository data encryption.

There is also some other legacy crypto stuff in low_level.pyx, but separating that would be a bit more effort as it affects also the build process, so guess we can keep that as is for now.