feat(tasks): assign folder='Tasks' at creation + backfill migration (#2834)

* feat: assign folder='Tasks' to task sessions at creation

Task sessions (LLM, action, research) now set folder='Tasks' on their
DbSession row, matching the pattern used by the Assistant folder. This
enables sidebar lens filtering without changing existing session
behaviour.

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

* feat: add backfill script for task session folders

One-shot script to set folder='Tasks' on existing [Task]/[Research]
sessions that predate the folder assignment in task_scheduler.py.

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

* refactor: replace standalone backfill script with automatic migration

Convert scripts/backfill_task_folders.py into _migrate_backfill_task_folders()
in core/database.py, called from init_db(). The migration is idempotent (only
touches rows where folder IS NULL/empty) and runs automatically on upgrade,
so operators no longer need a manual step to tag pre-existing task sessions.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

core/database.py

@@ -1631,6 +1631,33 @@ def init_db():
     _migrate_encrypt_email_passwords()
     _migrate_encrypt_signatures()
     _migrate_encrypt_endpoint_keys()
+    _migrate_backfill_task_folders()
+
+
+def _migrate_backfill_task_folders():
+    """Backfill folder='Tasks' on pre-existing task/research sessions.
+
+    Sessions created by the task scheduler (LLM tasks, action tasks, research
+    runs) now set folder='Tasks' at creation time.  This migration tags any
+    older sessions that predate that assignment.  Idempotent — only touches
+    rows where folder is NULL or empty and the title matches known prefixes.
+    """
+    try:
+        with engine.connect() as conn:
+            cols = [r[1] for r in conn.execute(text("PRAGMA table_info(sessions)"))]
+            if "folder" not in cols:
+                return
+            res = conn.execute(text(
+                "UPDATE sessions SET folder = 'Tasks' "
+                "WHERE (folder IS NULL OR folder = '') "
+                "AND (name LIKE '[Task] %' OR name LIKE '[Research] %')"
+            ))
+            conn.commit()
+            if res.rowcount:
+                logging.getLogger(__name__).info(
+                    f"Backfilled folder='Tasks' on {res.rowcount} task/research sessions")
+    except Exception as e:
+        logging.getLogger(__name__).warning(f"task folder backfill: {e}")
 
 
 def _migrate_chat_messages_fts():