crashdb_impl: add type annotations to memory backend

[Arnab-atra]

Summary

This pull request adds type annotations to apport/crashdb_impl/memory.py.

Changes

• Add type annotations throughout the module.
• Replace the explicit base class constructor call with super().
• Minor formatting improvements.
• No functional behavior changes.

Verification

I verified the changes with:

• python -m py_compile apport/crashdb_impl/memory.py
• ruff check apport/crashdb_impl/memory.py
• black --check apport/crashdb_impl/memory.py
• mypy apport/crashdb_impl/memory.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.46%. Comparing base (d9d3c9f) to head (096372b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #636      +/-   ##
==========================================
- Coverage   84.47%   84.46%   -0.01%     
==========================================
  Files         106      106              
  Lines       21053    21054       +1     
  Branches     3223     3223              
==========================================
  Hits        17784    17784              
- Misses       2792     2793       +1     
  Partials      477      477              

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