feat: Add mypy CI job and enforce stricter type checking

Overview

Added mypy type checking as a CI job and enabled stricter type checking configuration to catch potential type errors at lint time.

Changes

CI Integration

• .gitlab-ci.yml: Added mypy job in test stage
  • Runs uv run mypy on all backend code directories
  • Reuses existing before_script for dependency setup (no duplication)
  • Same trigger rules as other jobs (MRs, default branch, feature-final)

Configuration

• mypy.ini: Updated with stricter settings
  • warn_return_any = True - Warn when returning Any from typed functions
  • strict_optional = True - Handle Optional types more strictly
  • no_implicit_optional = True - Require explicit Optional for nullable params
  • warn_redundant_casts = True - Catch unnecessary type casts
  • warn_unused_ignores = True - Keep ignore comments clean
  • show_error_codes = True - Display error codes for better debugging
  • pretty = True - Formatted error output
  • Excluded tests/* from type checking

Bug Fix

• gitlab_utils/files_reader.py: Fixed type error
  • Added isinstance(content, bytes) check before decode
  • Returns proper str type instead of Any

Testing

Check	Status
ruff check .	✅ Passed
mypy .	✅ Passed (87 source files)

Behavior

• Pipeline: mypy now runs automatically on every push and MR
• Failure: CI fails if type errors are present
• Success: CI passes when all type checks pass
• No breaking changes: Existing pipeline behavior preserved