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/channels/constants.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.
+
+Channel constants that describe the platforms rather than how we talk to them.
+
+Separate from meta_base because these are shared by layers that have no
+business knowing about a transport — request schemas state one as a field
+default. (It buys no import weight: app/channels/__init__ loads the registry
+regardless. It is about where the value belongs, not what it drags in.)
+"""
+
+import enum
+from typing import Optional
+
+# The language assumed when a caller doesn't name one. Meta has no default —
+# name and language together identify a template, so `order_update` in five
+# languages is five templates — which makes this our choice, and one worth
+# keeping in a single place: four copies of the literal would silently disagree
+# the day it changes.
+#
+# The frontend's DEFAULT_LANGUAGE (utils/whatsappLanguages.ts) is the
+# unavoidable second copy, on the other side of the wire. Keep the two equal.
+DEFAULT_TEMPLATE_LANGUAGE = "en_US"
+
+
+class MessageSource(str, enum.Enum):
+    """Who wrote a message the business sent (chat_history.attributes.source)."""
+    AI = "ai"
+    HUMAN = "human"
+    SYSTEM = "system"   # canned notices: handover, limits, errors, tickets
+    APP = "app"         # an external app, through a channel extension
+
+
+# The most choices one reply can offer the customer: WhatsApp's list-message
+# row cap, the largest interactive form any channel supports.
+MAX_CHOICE_OPTIONS = 10
+# The longest one choice can be: a list row's description, which is where a
+# choice too long for a button or row title is shown in full.
+MAX_CHOICE_LENGTH = 72
+
+
+# Channels served by our own chat widget rather than an external messaging
+# platform. They render our UI, so widget-only features (the rating prompt) work
+# there; on Telegram or WhatsApp the same prompt is a dead end.
+#
+# 'shopify' is that widget embedded in a Shopify storefront — a distinct origin
+# worth reporting on, but the same chat surface. Every "is this the widget?"
+# test has to accept both, or Shopify conversations quietly lose widget
+# features the moment they stop being labelled 'web'.
+WIDGET_CHANNELS = frozenset({"web", "shopify"})
+
+
+def is_widget_channel(channel: Optional[str]) -> bool:
+    """True for our own widget surfaces. A missing channel means the widget."""
+    return not channel or channel in WIDGET_CHANNELS