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/rate_limiter_cleanup.py

@@ -8,15 +8,20 @@ consuming excessive memory.
 The cleanup is conservative: it only removes IPs with no recent attempts
 (all timestamps outside the rate-limit window), so active or recently-active
 IPs are preserved.
+
+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
 
 from app.tasks.timeout_utils import run_with_timeout
+from app.utils.correlation import get_correlation_id, reset_correlation_id, set_correlation_id
 
 if TYPE_CHECKING:
     from fastapi import FastAPI
@@ -35,7 +40,10 @@ JOB_ID: str = "rate_limiter_cleanup"
 TASK_TIMEOUT_SECONDS: int = 5
 
 
-async def _run_cleanup(app: FastAPI) -> None:
+async def _run_cleanup(
+    app: FastAPI,
+    correlation_id: str | None = None,
+) -> None:
     """Trigger cleanup of expired rate-limiter entries.
 
     Cleans up both the login-specific rate limiter (exponential backoff)
@@ -43,13 +51,27 @@ async def _run_cleanup(app: FastAPI) -> None:
 
     Args:
         app: The FastAPI application instance (holds the rate limiters).
+        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_app(app)
+    finally:
+        reset_correlation_id(token)
+
+
+async def _do_cleanup_with_app(app: FastAPI) -> None:
+    """Inner cleanup logic that runs with correlation context set."""
 
     async def _do_cleanup() -> None:
         login_limiter = getattr(app.state, "login_rate_limiter", None)
         if login_limiter is None:
             log.warning(
                 "rate_limiter_cleanup_skipped",
+                correlation_id=get_correlation_id(),
                 reason="login_rate_limiter not found on app.state",
             )
         else:
@@ -59,6 +81,7 @@ async def _run_cleanup(app: FastAPI) -> None:
         if global_limiter is None:
             log.warning(
                 "rate_limiter_cleanup_skipped",
+                correlation_id=get_correlation_id(),
                 reason="global_rate_limiter not found on app.state",
             )
         else: