Feat: chat channels — connect assistants to external messaging bots (#15850)

### What problem does this PR solve?

#15844

Adds a **Chat channels** capability so a RAGFlow assistant (Dialog) can
be exposed as a bot on external messaging platforms (Feishu/Lark,
Discord, Telegram, Slack, WeCom, LINE, etc.). An admin configures a bot
in the UI, connects it to an assistant, and inbound messages are
answered from that assistant's knowledge base — replies are delivered
back on the channel.

**Feishu/Lark is implemented and tested end-to-end.** Discord, Telegram,
LINE, and WeCom are scaffolded against the same interface; the remaining
listed channels are tracked as follow-ups.

### Design

**Backend**
- New `chat_channel` table (`tenant_id`, `name`, `channel`, `config`
JSON holding `{credential: {...}}`, `dialog_id`, `status`) +
`ChatChannelService` and RESTful CRUD under `/api/v1/chat_channels`.
- Channel framework under `api/channels/`: a `core` registry +
per-channel packages that self-register a builder and implement a common
`Channel` interface (`start`/`stop`/`send` + inbound normalization) over
`IncomingMessage`/`OutgoingMessage`.
- Embedded **reconcile loop** in `ragflow_server`
(`api/channels/bootstrap.py`): loads enabled bots, and
starts/stops/restarts them as rows change (no server restart needed).
Inbound messages run the connected dialog via the non-streaming
completion path, keeping per-end-user conversation history.
- Missing optional channel SDKs degrade gracefully (channel skipped with
a warning; others unaffected). Channel-level errors are logged, not
crashed.
- Feishu's WebSocket client runs in a dedicated thread with its own
event loop to avoid cross-loop/contextvars conflicts with the channel
runtime.

**Frontend**
- **Settings → Chat channels** panel: available-channels grid +
configured-bots list with add/edit/delete and a **Connect assistant**
popup that binds a bot to a dialog.
- Brand icons via simple-icons / reused shared data-source assets, with
colored fallbacks for brands not available.
- Route, sidebar entry, i18n (en/zh), and a top-nav segment-boundary fix
so the settings page no longer highlights the Chat tab.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### Notes
- DB: new `chat_channel` table is auto-created; `chat_channel.dialog_id`
is also covered by a `migrate_db` `alter_db_add_column` for existing
installs.
- Channel SDKs (`lark-oapi`, `discord.py`, `python-telegram-bot`,
`line-bot-sdk`, `wechatpy`, `aiohttp`) added to dependencies.
- Screenshots / per-channel credential docs to follow.

<img width="1338" height="1290" alt="Image"
src="https://github.com/user-attachments/assets/042cb2f9-0dad-4e6a-bcf7-43ced4bbd704"
/>

<img width="1344" height="738" alt="Image"
src="https://github.com/user-attachments/assets/373cd08e-ec40-4c67-9c51-4d948b1ba617"
/>

<img width="672" height="887" alt="Image"
src="https://github.com/user-attachments/assets/5a34953f-a9a3-4c1e-869e-5eff0dc64c84"
/>

---------

api/db/db_models.py

@@ -1215,6 +1215,22 @@ class Connector2Kb(DataBaseModel):
         db_table = "connector2kb"
 
 
+class ChatChannel(DataBaseModel):
+    id = CharField(max_length=32, primary_key=True)
+    tenant_id = CharField(max_length=32, null=False, index=True)
+    name = CharField(max_length=128, null=False, help_text="Bot name", index=False)
+    channel = CharField(max_length=128, null=False, help_text="Chat channel type", index=True)
+    config = JSONField(null=False, default={}, help_text="Channel credential & settings")
+    dialog_id = CharField(max_length=32, null=True, default=None, help_text="connected dialog id", index=True)
+    status = CharField(max_length=16, null=True, help_text="1: valid, 0: invalid", default="1", index=True)
+
+    def __str__(self):
+        return self.name
+
+    class Meta:
+        db_table = "chat_channel"
+
+
 class DateTimeTzField(CharField):
     field_type = 'VARCHAR'
 
@@ -1693,6 +1709,7 @@ def migrate_db():
     alter_db_add_column(migrator, "canvas_template", "canvas_category", CharField(max_length=32, null=False, default="agent_canvas", help_text="agent_canvas|dataflow_canvas", index=True))
     alter_db_add_column(migrator, "canvas_template", "canvas_types", ListField(null=True, default=list, help_text="Canvas types"))
     alter_db_add_column(migrator, "knowledgebase", "pipeline_id", CharField(max_length=32, null=True, help_text="Pipeline ID", index=True))
+    alter_db_add_column(migrator, "chat_channel", "dialog_id", CharField(max_length=32, null=True, help_text="connected dialog id", index=True))
     alter_db_add_column(migrator, "document", "pipeline_id", CharField(max_length=32, null=True, help_text="Pipeline ID", index=True))
     alter_db_add_column(migrator, "knowledgebase", "graphrag_task_id", CharField(max_length=32, null=True, help_text="Gragh RAG task ID", index=True))
     alter_db_add_column(migrator, "knowledgebase", "raptor_task_id", CharField(max_length=32, null=True, help_text="RAPTOR task ID", index=True))