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_flush.py

@@ -7,16 +7,21 @@ After Task 2 removed geo cache writes from GET requests, newly resolved IPs
 are only held in the in-memory cache until this task flushes them.  With the
 default 60-second interval, at most one minute of new resolution results is
 at risk on an unexpected process restart.
+
+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.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:
@@ -37,20 +42,37 @@ JOB_ID: str = "geo_cache_flush"
 TASK_TIMEOUT_SECONDS: int = 60
 
 
-async def _run_flush_with_resources(geo_cache: GeoCache, settings: Settings) -> None:
+async def _run_flush_with_resources(
+    geo_cache: GeoCache,
+    settings: Settings,
+    correlation_id: str | None = None,
+) -> None:
     """Flush the geo cache dirty set to the application database.
 
     Args:
         geo_cache: The application's GeoCache instance.
         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_flush_with_resources(geo_cache, settings)
+    finally:
+        reset_correlation_id(token)
+
+
+async def _do_flush_with_resources(geo_cache: GeoCache, settings: Settings) -> None:
+    """Inner flush logic that runs with correlation context set."""
 
     async def _do_flush() -> None:
         async with task_db(settings) as db:
             count = await geo_cache.flush_dirty(db)
 
         if count > 0:
-            log.debug("geo_cache_flush_ran", flushed=count)
+            log.debug("geo_cache_flush_ran", correlation_id=get_correlation_id(), flushed=count)
 
     await run_with_timeout("geo_cache_flush", _do_flush(), TASK_TIMEOUT_SECONDS)