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>

.pre-commit-config.yaml

@@ -29,15 +29,9 @@ repos:
       - id: check-merge-conflict
       - id: debug-statements
 
-  # Python formatting (optional - uncomment if you want)
-  # - repo: https://github.com/psf/black
-  #   rev: 23.12.1
-  #   hooks:
-  #     - id: black
-  #       language_version: python3
-
-  # Python import sorting (optional)
-  # - repo: https://github.com/pycqa/isort
-  #   rev: 5.13.2
-  #   hooks:
-  #     - id: isort
+  # Ruff - linting and import sorting (replaces black + isort)
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.8.4
+    hooks:
+      - id: ruff
+        args: [--fix, --exit-non-zero-on-fix]