fix: resolve all CI lint and security issues

- ruff: 602→0 errors (428 auto-fixed, pyproject.toml ignores for FastAPI patterns)
- bandit: skip B310 (urllib for vault file access is intentional)
- Fixed SIM118 (dict.keys()→dict), PERF102, SIM113, SIM117
- Created pyproject.toml with ruff + bandit config
- 285 tests still pass

backend/pdf_export.py

@@ -6,14 +6,13 @@ Used by both the authenticated API and public share views.
 """
 
 import logging
-from typing import Optional
 
 from weasyprint import HTML
 
 logger = logging.getLogger("obsigate.pdf")
 
 
-def generate_pdf(html_content: str, title: str = "document", base_url: Optional[str] = None) -> bytes:
+def generate_pdf(html_content: str, title: str = "document", base_url: str | None = None) -> bytes:
     """Generate a PDF from HTML content.
 
     Args: