feat: Add external chat gateways for Telegram, WhatsApp, Slack, and Discord

[AnishSarkar22]

Description

• Added the external chat gateway persistence layer for platform accounts, conversation bindings, inbound inbox events, and canonical chat thread/message metadata.
• Implemented the Telegram gateway path, including pairing, webhook intake, long-poll/BYO intake modes, command handling, stream translation, inbox processing, worker maintenance, and settings UI.
• Refactored gateway internals into provider-neutral abstractions for commands, formatting, platform bundles, inbox processing, request context, metrics, and external chat routing.
• Added WhatsApp support through both WhatsApp Cloud API and self-hosted Baileys bridge flows, including Cloud webhooks, bridge service, pairing endpoints, Docker/runtime wiring, and messaging channel controls.
• Added Slack gateway support with workspace-scoped accounts, app mention parsing, threaded replies, OAuth/webhook flow, Slack command handling, maintenance integration, and settings UI.
• Added Discord gateway support with guild-scoped accounts, bot mention intake supervisor, REST send/update support, install flow, channel replies, maintenance integration, and settings UI.
• Added gateway observability and operational safeguards, including queue/inbox metrics, webhook parse metrics, rate-limit handling, HITL fallbacks, event stream cleanup, and maintenance health checks.
• Added docs for all 4 gateways.

Motivation and Context

FIX #

Screenshots

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR integrates external messaging channels (Telegram and WhatsApp) into SurfSense. The implementation creates a gateway infrastructure that routes messages from external platforms through the same canonical chat agent used by the web UI. It adds database tables for managing external chat accounts, bindings, and inbound event queues, platform-specific adapters for Telegram and WhatsApp (both Meta Cloud API and self-hosted Baileys bridge), and an in-process FastAPI inbox worker that processes external messages and invokes the chat agent. The architecture supports webhook-based intake for production (Telegram, WhatsApp Cloud) and long-polling fallback for self-hosted deployments. The PR also includes a WhatsApp bridge Node.js service using Baileys for self-hosted WhatsApp integration, web UI components for pairing/managing channels, comprehensive observability metrics, and unit tests for core gateway components.

⏱️ Estimated Review Time: 1-3 hours

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/app/config/__init__.py
2	surfsense_backend/app/db.py
3	surfsense_backend/alembic/versions/144_add_gateway_tables.py
4	surfsense_backend/app/gateway/base/adapter.py
5	surfsense_backend/app/gateway/base/translator.py
6	surfsense_backend/app/gateway/base/commands.py
7	surfsense_backend/app/gateway/telegram/adapter.py
8	surfsense_backend/app/gateway/telegram/translator.py
9	surfsense_backend/app/gateway/telegram/commands.py
10	surfsense_backend/app/gateway/whatsapp/adapter_cloud.py
11	surfsense_backend/app/gateway/whatsapp/adapter_baileys.py
12	surfsense_backend/app/gateway/whatsapp/translator.py
13	surfsense_backend/app/gateway/whatsapp/translator_baileys.py
14	surfsense_backend/app/gateway/whatsapp/commands.py
15	surfsense_backend/app/gateway/registry.py
16	surfsense_backend/app/gateway/inbox.py
17	surfsense_backend/app/gateway/inbox_processor.py
18	surfsense_backend/app/gateway/inbox_worker.py
19	surfsense_backend/app/gateway/agent_invoke.py
20	surfsense_backend/app/gateway/pairing.py
21	surfsense_backend/app/gateway/bindings.py
22	surfsense_backend/app/gateway/accounts.py
23	surfsense_backend/app/gateway/runner.py
24	surfsense_backend/app/gateway/byo_long_poll.py
25	surfsense_backend/app/routes/gateway_webhook_routes.py
26	surfsense_backend/app/routes/gateway_whatsapp_webhook_routes.py
27	surfsense_backend/app/routes/gateway_whatsapp_baileys_routes.py
28	surfsense_backend/app/routes/__init__.py
29	surfsense_backend/app/tasks/celery_tasks/gateway_tasks.py
30	surfsense_backend/app/celery_app.py
31	surfsense_backend/app/app.py
32	surfsense_backend/app/observability/metrics.py
33	surfsense_backend/scripts/whatsapp-bridge/bridge.js
34	surfsense_backend/scripts/whatsapp-bridge/Dockerfile
35	surfsense_backend/scripts/whatsapp-bridge/package.json
36	docker/docker-compose.yml
37	surfsense_backend/.env.example
38	surfsense_backend/pyproject.toml
39	surfsense_backend/scripts/docker/entrypoint.sh
40	surfsense_backend/scripts/register_webhook.py
41	surfsense_web/app/dashboard/[search_space_id]/user-settings/components/MessagingChannelsContent.tsx
42	surfsense_web/app/dashboard/[search_space_id]/user-settings/layout-shell.tsx
43	surfsense_web/app/dashboard/[search_space_id]/user-settings/messaging-channels/page.tsx
44	surfsense_web/.env.example
45	surfsense_web/zero/schema/chat.ts
46	surfsense_web/content/docs/manual-installation.mdx
47	surfsense_backend/tests/unit/gateway/test_formatting.py
48	surfsense_backend/tests/unit/gateway/test_hitl_filter.py
49	surfsense_backend/tests/unit/gateway/test_pairing.py
50	surfsense_backend/tests/unit/gateway/test_webhook_routes.py
51	surfsense_backend/tests/unit/gateway/test_inbox_worker.py
52	surfsense_backend/tests/unit/gateway/test_byo_long_poll_lifespan.py
53	surfsense_backend/tests/unit/gateway/test_enqueue_received_sweep.py
54	surfsense_backend/tests/unit/gateway/test_process_inbound_event_task.py