feat(modules): 帮助中心、工单、工作流与多渠道接入

补齐 FAQ、帮助中心、工单、可视化工作流及 Telegram/WhatsApp/Slack 等渠道模块。

backend/app/services/ticket_access.py

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+"""
+Copyright 2024-2026 ReplyPier
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+AI-ticketing plan gating: paid-plan feature on cloud, unrestricted for
+OSS/self-hosted. Thin wrappers over the generic feature gate (the
+help-center pattern).
+"""
+
+from uuid import UUID
+
+from sqlalchemy.orm import Session
+
+from app.services.feature_gate import check_feature_access, feature_allowed
+
+AI_TICKETING_FEATURE = "ai_ticketing"
+
+UPGRADE_MESSAGE = (
+    "AI Ticketing is not available in your current plan. "
+    "Please upgrade to access this feature."
+)
+
+
+def ticketing_allowed(db: Session, organization_id: UUID) -> bool:
+    """Non-raising check — used by the chat-agent tool assembly and workers.
+    Fails closed on gating errors."""
+    return feature_allowed(db, organization_id, AI_TICKETING_FEATURE)
+
+
+def check_ticketing_access(db: Session, organization_id: UUID) -> None:
+    """Raising check for admin endpoints (403 with the upgrade message)."""
+    check_feature_access(db, organization_id, AI_TICKETING_FEATURE, UPGRADE_MESSAGE)