refactor: modernize code quality tooling with Ruff

- Replace black, isort, and flake8 with Ruff (all-in-one linter and formatter)
- Add comprehensive pyproject.toml configuration
- Simplify Makefile code quality targets
- Configure exclusions for venv/.venv in pyproject.toml
- Auto-fix 1,359 linting issues across codebase

Benefits:
- Much faster builds (Ruff is written in Rust)
- Single tool replaces multiple tools
- More comprehensive rule set (UP, B, C4, SIM, PIE, RET, Q)
- All configuration centralized in pyproject.toml
- Better import sorting and formatting consistency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/api/v1/admin/dashboard.py

@@ -4,7 +4,6 @@ Admin dashboard and statistics endpoints.
 """
 
 import logging
-from typing import List
 
 from fastapi import APIRouter, Depends
 from sqlalchemy.orm import Session
@@ -57,7 +56,7 @@ def get_comprehensive_stats(
     )
 
 
-@router.get("/stats/marketplace", response_model=List[MarketplaceStatsResponse])
+@router.get("/stats/marketplace", response_model=list[MarketplaceStatsResponse])
 def get_marketplace_stats(
     db: Session = Depends(get_db),
     current_admin: User = Depends(get_current_admin_api),