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 84.49%. Comparing base (2eacb11) to head (72726bc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #636   +/-   ##
=======================================
  Coverage   84.48%   84.49%           
=======================================
  Files         106      106           
  Lines       21018    21019    +1     
  Branches     3195     3195           
=======================================
+ Hits        17758    17760    +2     
  Misses       2785     2785           
+ Partials      475      474    -1     

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