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/session_cleanup.py

@@ -6,10 +6,14 @@ months of operation and degrades query performance.
 
 Individual expired sessions are removed on-demand when validated, but the bulk
 cleanup ensures comprehensive pruning at a predictable interval.
+
+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 uuid
 from typing import TYPE_CHECKING
 
 import structlog
@@ -17,6 +21,7 @@ import structlog
 from app.repositories import session_repo
 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
 from app.utils.time_utils import utc_now
 
@@ -37,19 +42,40 @@ JOB_ID: str = "session_cleanup"
 TASK_TIMEOUT_SECONDS: int = 30
 
 
-async def _run_cleanup_with_resources(settings: Settings) -> None:
+async def _run_cleanup_with_resources(
+    settings: Settings,
+    correlation_id: str | None = None,
+) -> None:
     """Delete all expired sessions from the database.
 
     Args:
         settings: The resolved application settings used for database access.
+        correlation_id: Optional correlation ID from the triggering request.
     """
+    if correlation_id is None:
+        correlation_id = str(uuid.uuid4())
+
+    token = set_correlation_id(correlation_id)
+    try:
+        await _do_cleanup_with_settings(settings)
+    finally:
+        reset_correlation_id(token)
+
+
+async def _do_cleanup_with_settings(settings: Settings) -> None:
+    """Inner cleanup logic that runs with correlation context set."""
 
     async def _do_cleanup() -> None:
         now_iso = utc_now().isoformat()
         async with task_db(settings) as db:
             deleted_count = await session_repo.delete_expired_sessions(db, now_iso)
 
-        log.info("session_cleanup_ran", deleted_count=deleted_count, cutoff_time=now_iso)
+        log.info(
+            "session_cleanup_ran",
+            correlation_id=get_correlation_id(),
+            deleted_count=deleted_count,
+            cutoff_time=now_iso,
+        )
 
     await run_with_timeout("session_cleanup", _do_cleanup(), TASK_TIMEOUT_SECONDS)