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/correlation.py

@@ -0,0 +1,70 @@
+"""Correlation ID context variable for distributed tracing.
+
+This module provides a :class:`contextvars.ContextVar` that stores the correlation
+ID for the current execution context. Background tasks inherit or explicitly set
+the correlation ID to enable log correlation across request/task boundaries.
+
+Usage in background tasks::
+
+    from app.utils.correlation import get_correlation_id, set_correlation_id
+
+    async def my_background_task(correlation_id: str) -> None:
+        token = set_correlation_id(correlation_id)
+        try:
+            log.info("task_started")  # Logs include correlation_id
+        finally:
+            reset_correlation_id(token)
+
+For APScheduler jobs, pass the correlation ID through kwargs and call
+:func:`set_correlation_id` at the start of the async callback.
+"""
+
+from __future__ import annotations
+
+from contextvars import ContextVar
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from contextvars import Token
+
+#: Default correlation ID used when no parent request context exists.
+_DEFAULT_BG_CORRELATION_ID: str = "bg-task"
+
+#: ContextVar holding the current correlation ID.
+correlation_id_var: ContextVar[str] = ContextVar(
+    "correlation_id",
+    default=_DEFAULT_BG_CORRELATION_ID,
+)
+
+
+def get_correlation_id() -> str:
+    """Return the current correlation ID from context.
+
+    Returns:
+        The current correlation ID string. Returns ``"bg-task"`` if no
+        correlation ID has been set in the current context.
+    """
+    return correlation_id_var.get()
+
+
+def set_correlation_id(correlation_id: str) -> Token[str]:
+    """Set the correlation ID for the current context.
+
+    Args:
+        correlation_id: The correlation ID to bind (typically passed from
+            the triggering request).
+
+    Returns:
+        A :class:`contextvars.Token` that must be passed to
+        :func:`reset_correlation_id` when the task completes.
+    """
+    return correlation_id_var.set(correlation_id)
+
+
+def reset_correlation_id(token: Token[str]) -> None:
+    """Reset the correlation ID to its previous value.
+
+    Args:
+        token: The token returned by :func:`set_correlation_id`.
+    """
+    correlation_id_var.reset(token)