refactor(docs+comments): add Google-style docstrings and inline comments across backend

Task D — Google-style docstrings (Args/Returns) on every public function,
method, and class across all 158 Python files in the backend. Zero ruff D
violations (pydocstyle Google convention).

Task E — Explanatory one-line comment before every code line (~11600 new
comments). ruff check passes clean after isort re-sort.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/jobs/jira_sync_job.py

@@ -1,37 +1,65 @@
 """Scheduled job — syncs all Jira links hourly."""
 
+# Import logging
 import logging
 
+# Import settings from app.config
 from app.config import settings
+
+# Import SessionLocal from app.database
 from app.database import SessionLocal
+
+# Import JiraLink from app.models.jira_link
 from app.models.jira_link import JiraLink
+
+# Import jira_service from app.services
 from app.services import jira_service
 
+# Assign logger = logging.getLogger(__name__)
 logger = logging.getLogger(__name__)
 
 
+# Define function sync_all_jira_links
 def sync_all_jira_links() -> None:
     """Pull latest status from Jira for every stored link.
 
     Silently skips if ``JIRA_ENABLED`` is ``False``.  Individual link
     failures are logged but do not abort the rest of the batch.
     """
+    # Check: not settings.JIRA_ENABLED
     if not settings.JIRA_ENABLED:
+        # Return control to caller
         return
 
+    # Assign db = SessionLocal()
     db = SessionLocal()
+    # Attempt the following; catch errors below
     try:
+        # Assign links = db.query(JiraLink).all()
         links = db.query(JiraLink).all()
+        # Assign synced = 0
         synced = 0
+        # Iterate over links
         for link in links:
+            # Attempt the following; catch errors below
             try:
+                # Call jira_service.sync_jira_to_aegis()
                 jira_service.sync_jira_to_aegis(db, link)
+                # Assign synced = 1
                 synced += 1
+            # Handle Exception
             except Exception as e:
+                # Log warning: "Jira sync failed for link %s: %s", link.id, e
                 logger.warning("Jira sync failed for link %s: %s", link.id, e)
+        # Commit all pending changes to the database
         db.commit()
+        # Log info: "Jira sync completed: %d/%d links updated", synced
         logger.info("Jira sync completed: %d/%d links updated", synced, len(links))
+    # Handle Exception
     except Exception:
+        # Log exception: "Jira sync batch job failed"
         logger.exception("Jira sync batch job failed")
+    # Always execute this cleanup block
     finally:
+        # Close the database session
         db.close()