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/tasks/history_sync.py

@@ -2,11 +2,15 @@
 
 Periodically copies new records from the fail2ban sqlite database into the
 BanGUI application archive table to prevent gaps when fail2ban purges old rows.
+
+Correlation IDs are propagated through the task using :mod:`app.utils.correlation`
+so that task logs can be correlated across runs.
 """
 
 from __future__ import annotations
 
 import datetime
+import uuid
 from typing import TYPE_CHECKING
 
 import structlog
@@ -14,6 +18,7 @@ import structlog
 from app.services import history_service
 from app.tasks.db import task_db
 from app.tasks.timeout_utils import run_with_timeout
+from app.utils.correlation import get_correlation_id, reset_correlation_id, set_correlation_id
 from app.utils.runtime_state import get_effective_settings
 
 if TYPE_CHECKING:
@@ -36,16 +41,32 @@ BACKFILL_WINDOW: int = 648000
 TASK_TIMEOUT_SECONDS: int = 60
 
 
-async def _run_sync_with_settings(settings: Settings) -> None:
+async def _run_sync_with_settings(
+    settings: Settings,
+    correlation_id: str | None = None,
+) -> None:
+    """Run the history sync with correlation ID context."""
+    if correlation_id is None:
+        correlation_id = str(uuid.uuid4())
+
+    token = set_correlation_id(correlation_id)
+    try:
+        await _do_sync_with_settings(settings)
+    finally:
+        reset_correlation_id(token)
+
+
+async def _do_sync_with_settings(settings: Settings) -> None:
+    """Inner sync logic that runs with correlation context set."""
     socket_path: str = settings.fail2ban_socket
 
     async def _do_sync() -> None:
         try:
             async with task_db(settings) as db:
                 synced = await history_service.sync_from_fail2ban_db(db, socket_path)
-            log.info("history_sync_complete", synced=synced)
+            log.info("history_sync_complete", correlation_id=get_correlation_id(), synced=synced)
         except Exception:
-            log.exception("history_sync_failed")
+            log.exception("history_sync_failed", correlation_id=get_correlation_id())
 
     await run_with_timeout("history_sync", _do_sync(), TASK_TIMEOUT_SECONDS)