feat: Integration API — proxy endpoint for external workflow triggers

[morgan-wowk]

Summary

• Adds POST /api/integration/slack_workflow_trigger — a server-side proxy that accepts a logical workflow identifier + arbitrary payload, resolves the target webhook URL from server configuration, and forwards the payload.
• Keeps sensitive webhook URLs out of frontend config entirely; only a logical workflowId is needed in the client.
• Route is only registered when at least one workflow URL is configured (TANGLE_SLACK_WORKFLOW_URLS env var, a JSON object mapping workflow IDs to URLs). The endpoint is absent (404) in unconfigured OSS deployments.
• api_server_main.py reads TANGLE_SLACK_WORKFLOW_URLS from the environment (JSON string), parses it, and passes the mapping to setup_routes().

New files

• cloud_pipelines_backend/integration_api.py — IntegrationApiService + register_routes()

Modified files

• cloud_pipelines_backend/api_router.py — threads slack_workflow_urls through setup_routes() → _setup_routes_internal(), calls integration_api.register_routes() before app.include_router()
• api_server_main.py — reads + parses TANGLE_SLACK_WORKFLOW_URLS env var, passes to setup_routes()

Test plan

• Start server with TANGLE_SLACK_WORKFLOW_URLS={} — confirm /api/integration/slack_workflow_trigger returns 404
• Start server with TANGLE_SLACK_WORKFLOW_URLS={"test_wf": "https://example.com/hook"} — confirm POST with {"workflow_internal_id": "test_wf", "webhook_payload": {}} routes to the URL
• Confirm unknown workflow_internal_id returns 404 with descriptive message
• Confirm unreachable URL returns 502

🤖 Generated with Claude Code

[morgan-wowk]

• feat: Integration API — proxy endpoint for external workflow triggers #284 👈 (View in Graphite)
• master

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[morgan-wowk]

Moving this implementation to Shopify/oasis-backend instead — the Slack proxy endpoint will be Shopify-internal only for now.