fix(migrations): add migration lock and batch DB updates

[ReenigneArcher]

Description

Introduce a global MIGRATION_LOCK to prevent concurrent migrations and refactor Reddit/Mee6 import flows into internal methods. Migrations now disable database.GIT_ENABLED and restore its original state on completion or error. Reddit migration applies updates in a single transactional context by building an existing-users map and performing inserts/updates with progress logging. Mee6 migration collects updates asynchronously and applies them in batched transactions, with improved error handling and final DB sync.

Screenshot

Issues Fixed or Closed

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[codecov]

Codecov Report

❌ Patch coverage is 35.29412% with 88 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.96%. Comparing base (d56d2ad) to head (c90533d).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/common/rank.py	35.29%	88 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
+ Coverage   59.45%   59.96%   +0.51%     
==========================================
  Files          24       24              
  Lines        1761     1806      +45     
==========================================
+ Hits         1047     1083      +36     
- Misses        714      723       +9     

Files with missing lines	Coverage Δ
src/common/rank.py	58.27% <35.29%> (+4.19%)	⬆️

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud