Lint cleanup: ruff fixes across entire codebase

- Strip trailing whitespace from all Python files
- Fix import sorting (I001) across all modules
- Convert Optional[X] to X | None syntax (UP045)
- Remove unused imports (F401)
- Convert lambda assignments to def functions (E731)
- Add TYPE_CHECKING import for forward references
- Update pyproject.toml ruff config:
  - Move select/ignore to [tool.ruff.lint] section
  - Add per-file ignores for DCT colorspace naming (N803/N806)
  - Add per-file ignores for __init__.py import structure (E402)
  - Exclude defunct test_routes.py
- Remove frontends/web/test_routes.py (defunct debug snippet)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pyproject.toml

@@ -114,9 +114,18 @@ target-version = ["py310", "py311", "py312"]
 
 [tool.ruff]
 line-length = 100
+exclude = ["frontends/web/test_routes.py"]  # Debug snippet, not a real module
+
+[tool.ruff.lint]
 select = ["E", "F", "I", "N", "W", "UP"]
 ignore = ["E501"]
 
+[tool.ruff.lint.per-file-ignores]
+# YCbCr colorspace variables (R, G, B, Y, Cb, Cr) are standard names
+"src/stegasoo/dct_steganography.py" = ["N803", "N806"]
+# Package __init__.py has imports after try/except and aliases - intentional structure
+"src/stegasoo/__init__.py" = ["E402"]
+
 [tool.mypy]
 python_version = "3.10"
 warn_return_any = true