feat(channels): add Google Chat channel with Pub/Sub pull and Cards V2

[duhd-vnpay]

Summary

• New Google Chat channel implementation with Pub/Sub pull mode for receiving messages
• Cards V2 rich formatting with markdown-to-card conversion
• Drive upload for long-form responses (configurable threshold)
• Streaming: progressive message editing for DMs and groups (configurable per type)
• Service account auth with auto-refresh, media handling (download + Drive upload)
• DM/group policy controls, @mention toggle, allowlist support
• Full Web Dashboard integration: credentials form, config schema, channel type selector

Architecture

internal/channels/googlechat/
├── googlechat.go      # Channel struct, lifecycle (Start/Stop)
├── factory.go         # Channel instance factory (DB-driven)
├── auth.go            # Service account JWT + token refresh
├── pubsub.go          # Pub/Sub pull loop + message routing
├── webhook.go         # Webhook stub (phase 2)
├── send.go            # Message sending (text, cards, Drive)
├── format.go          # Markdown → Cards V2 conversion
├── media.go           # Media download/upload (Chat API + Drive)
├── stream.go          # Streaming (edit-in-place with throttle)
├── constants.go       # Config defaults, API URLs
└── *_test.go          # Unit tests (auth, format, pubsub, send, stream)

No new Go dependencies — uses existing google/uuid, net/http, golang.org/x/oauth2.

Changes (v2 — rebased on latest main)

• Rebased on upstream main (32ced98) to resolve all conflicts
• Cleaned up: only Google Chat changes, no unrelated modifications
• 20 files changed, 3525 insertions

Config example

{
  "channels": {
    "google_chat": {
      "enabled": true,
      "serviceAccountFile": "/app/config/google-sa.json",
      "mode": "pubsub",
      "projectId": "my-project",
      "subscriptionId": "my-chat-sub",
      "pullIntervalMs": 3000,
      "dm_stream": true,
      "group_stream": false
    }
  }
}

Test plan

• go build ./cmd/ passes (Linux target)
• Unit tests in internal/channels/googlechat/
• Manual test with GCP service account + Pub/Sub subscription
• Verify Cards V2 rendering in Google Chat client