Add audit logging, rate limiting, secret redactor, and backlinks

Implement several security and feature improvements across the backend
and frontend:
- New IP-based rate limiter for authentication endpoints
- New audit logging system for sensitive operations
- New secret redactor to mask sensitive patterns in rendered content
- Configurable token TTL and IGNORED_DIRS via environment variables
- Add backlink index and API endpoint
- Add preview tab support with single/double-click behavior in tree
- Add file backup before write/delete operations

backend/auth/jwt_handler.py

@@ -19,8 +19,8 @@ SECRET_KEY_FILE = Path("data/secret.key")
 REVOKED_TOKENS_FILE = Path("data/revoked_tokens.json")
 
 ALGORITHM = "HS256"
-ACCESS_TOKEN_EXPIRE_SECONDS = 3600      # 1 hour
-REFRESH_TOKEN_EXPIRE_SECONDS = 604800   # 7 days
+ACCESS_TOKEN_EXPIRE_SECONDS = int(os.environ.get("OBSIGATE_ACCESS_TOKEN_TTL", "3600"))       # default 1 hour
+REFRESH_TOKEN_EXPIRE_SECONDS = int(os.environ.get("OBSIGATE_REFRESH_TOKEN_TTL", "604800"))   # default 7 days
 
 # In-memory revoked token set (loaded from disk on startup)
 _revoked_jtis: set = set()