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/geo_cache_cleanup.py

@@ -7,10 +7,14 @@ database file and maintains query performance on geo lookups.
 
 When a stale IP is encountered again after purge, it will be re-resolved from
 the MaxMind database or ip-api.com (if configured), which is acceptable.
+
+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 datetime import UTC, datetime, timedelta
 from typing import TYPE_CHECKING
 
@@ -19,6 +23,7 @@ import structlog
 from app.repositories import geo_cache_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
 
 if TYPE_CHECKING:
@@ -41,7 +46,10 @@ JOB_ID: str = "geo_cache_cleanup"
 TASK_TIMEOUT_SECONDS: int = 60
 
 
-async def _run_cleanup_with_resources(settings: Settings) -> None:
+async def _run_cleanup_with_resources(
+    settings: Settings,
+    correlation_id: str | None = None,
+) -> None:
     """Delete stale entries from the geo cache.
 
     Calculates a cutoff timestamp (now - retention period) and removes all
@@ -49,7 +57,20 @@ async def _run_cleanup_with_resources(settings: Settings) -> None:
 
     Args:
         settings: The resolved application settings used for database access.
+        correlation_id: Optional correlation ID for log correlation.
     """
+    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:
         cutoff_dt = datetime.now(UTC) - timedelta(days=GEO_CACHE_RETENTION_DAYS)
@@ -60,9 +81,19 @@ async def _run_cleanup_with_resources(settings: Settings) -> None:
             await db.commit()
 
         if deleted > 0:
-            log.info("geo_cache_cleanup_ran", deleted=deleted, retention_days=GEO_CACHE_RETENTION_DAYS)
+            log.info(
+                "geo_cache_cleanup_ran",
+                correlation_id=get_correlation_id(),
+                deleted=deleted,
+                retention_days=GEO_CACHE_RETENTION_DAYS,
+            )
         else:
-            log.debug("geo_cache_cleanup_ran", deleted=deleted, retention_days=GEO_CACHE_RETENTION_DAYS)
+            log.debug(
+                "geo_cache_cleanup_ran",
+                correlation_id=get_correlation_id(),
+                deleted=deleted,
+                retention_days=GEO_CACHE_RETENTION_DAYS,
+            )
 
     await run_with_timeout("geo_cache_cleanup", _do_cleanup(), TASK_TIMEOUT_SECONDS)