Update observability docs and task utilities

- Add Observability.md documentation
- Standardize task logging with correlation_id support
- Add log_sanitizer utility for PII masking
- Update Tasks.md tracking
- Update geo_cache tasks and other task modules with correlation_id

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/utils/config_file_utils.py

@@ -29,6 +29,7 @@ from app.utils.fail2ban_client import (
     Fail2BanResponse,
 )
 from app.utils.fail2ban_response import ok, to_dict
+from app.utils.log_sanitizer import sanitize_for_logging
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -500,8 +501,8 @@ async def start_daemon(start_cmd_parts: list[str]) -> bool:
             "fail2ban_start_failed",
             command=" ".join(start_cmd_parts),
             returncode=process.returncode,
-            stdout=stdout.decode("utf-8", errors="replace"),
-            stderr=stderr.decode("utf-8", errors="replace"),
+            stdout=sanitize_for_logging(stdout.decode("utf-8", errors="replace")),
+            stderr=sanitize_for_logging(stderr.decode("utf-8", errors="replace")),
         )
         return False
     log.info(