Add Slack interactivity webhook support

[suhaibmujahid]

Resolves #6279

Add a dedicated Slack interactions endpoint with request signature verification.

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[Copilot]

Pull request overview

Adds inbound Slack interactivity support to hackbot-api by introducing a dedicated /slack/interactions endpoint protected with Slack request signature verification, plus parsing of Slack’s form-encoded interaction payloads into a normalized click object.

Changes:

• Introduces Slack signature verification (require_slack_signature / verify_slack_signature) and wires it into a new Slack router.
• Adds Slack interaction payload parsing utilities (parse_interaction / parse_payload) and registers the new router in the FastAPI app.
• Adds slack-sdk dependency and ensures test env defaults include SLACK_SIGNING_SECRET.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
services/hackbot-api/tests/conftest.py	Adds dummy SLACK_SIGNING_SECRET so settings validation doesn’t block test imports.
services/hackbot-api/pyproject.toml	Adds slack-sdk dependency for signature verification.
services/hackbot-api/app/slack_webhook.py	New Slack interaction payload decoding/parsing into a ButtonClick model.
services/hackbot-api/app/routers/slack.py	New /slack/interactions endpoint secured by Slack HMAC signature verification.
services/hackbot-api/app/routers/init.py	Exposes the new Slack router from the router package.
services/hackbot-api/app/main.py	Registers the Slack router on the FastAPI app.
services/hackbot-api/app/config.py	Adds Slack settings model and embeds it into global settings.
services/hackbot-api/app/auth.py	Implements Slack signature verification + dependency for request authentication.

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[evgenyrp]

Could you also ask Claude to document this functionality concisely in /docs/hackbot? This would also help with review.