fix(lint): pin ruff version, add pre-commit hook, fix all lint errors

- Pin ruff==0.8.4 in requirements-dev.txt (was >=0.8.4, CI got newer
  version with different import sorting rules)
- Add ruff to .pre-commit-config.yaml with --fix to auto-sort imports
  on commit (prevents PyCharm import reordering from reaching CI)
- Fix I001 import sorting in 6 files
- Fix F401 unused import (sqlalchemy.Numeric in subscription.py)
- Fix noqa false positive in validate_architecture.py comment

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements-dev.txt

@@ -5,7 +5,7 @@ alembic>=1.14.0
 # Linting and formatting tools
 black>=24.10.0
 isort>=5.13.2
-ruff>=0.8.4  # Modern alternative to flake8, faster and Python 3.13 compatible
+ruff==0.8.4  # Pinned — must match pre-commit hook and local version
 mypy>=1.13.0
 
 # Optional: More advanced linting (if you prefer flake8 over ruff)