fix: add graceful degradation for DB errors in tasks.py

Wrap all _get_db() calls with sqlite3.OperationalError handling so
endpoints degrade gracefully when the task DB is locked or missing:
- Read endpoints (page, partials, list, queue status) return empty data
- Write endpoints (create, update, delete) return 503

Fixes #943

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

tests/dashboard/test_tasks_api.py

@@ -3,6 +3,99 @@
 Verifies task CRUD operations and the dashboard page rendering.
 """
 
+import sqlite3
+from unittest.mock import patch
+
+
+# ---------------------------------------------------------------------------
+# DB error handling tests
+# ---------------------------------------------------------------------------
+
+_DB_ERROR = sqlite3.OperationalError("database is locked")
+
+
+def test_tasks_page_degrades_on_db_error(client):
+    """GET /tasks renders empty columns when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.get("/tasks")
+    assert response.status_code == 200
+    assert "TASK QUEUE" in response.text
+
+
+def test_pending_partial_degrades_on_db_error(client):
+    """GET /tasks/pending returns fallback HTML when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.get("/tasks/pending")
+    assert response.status_code == 200
+    assert "Database unavailable" in response.text
+
+
+def test_active_partial_degrades_on_db_error(client):
+    """GET /tasks/active returns fallback HTML when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.get("/tasks/active")
+    assert response.status_code == 200
+    assert "Database unavailable" in response.text
+
+
+def test_completed_partial_degrades_on_db_error(client):
+    """GET /tasks/completed returns fallback HTML when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.get("/tasks/completed")
+    assert response.status_code == 200
+    assert "Database unavailable" in response.text
+
+
+def test_api_create_task_503_on_db_error(client):
+    """POST /api/tasks returns 503 when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.post("/api/tasks", json={"title": "Test"})
+    assert response.status_code == 503
+
+
+def test_api_list_tasks_empty_on_db_error(client):
+    """GET /api/tasks returns empty list when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.get("/api/tasks")
+    assert response.status_code == 200
+    assert response.json() == []
+
+
+def test_queue_status_degrades_on_db_error(client):
+    """GET /api/queue/status returns idle status when DB is unavailable."""
+    with patch(
+        "dashboard.routes.tasks._get_db",
+        side_effect=_DB_ERROR,
+    ):
+        response = client.get("/api/queue/status")
+    assert response.status_code == 200
+    data = response.json()
+    assert data["is_working"] is False
+    assert data["current_task"] is None
+
+
+# ---------------------------------------------------------------------------
+# Existing tests
+# ---------------------------------------------------------------------------
+
 
 def test_tasks_page_returns_200(client):
     response = client.get("/tasks")