feat: add Slack channel import as a document source

[cameronhotchkies]

Stacked on #12 (refactor PR).

ref: #8

What

This adds Slack as a document source alongside PDFs and markdown. Users can point frfr at a Slack channel by ID or URL, and it fetches messages and threads via the Slack Web API, cleans up Slack mrkdwn into plain text (user mentions, channel refs, links, emoji, HTML entities), writes the result to a text file, and feeds it through the existing fact extraction pipeline. A new React modal handles channel input with optional date filtering, and high-volume channels are automatically limited to a configurable lookback window (default 90 days). Requires a SLACK_BOT_TOKEN env var.

Usage

# Add a Slack channel to a session
curl -X POST http://localhost:8080/api/sessions/$SESSION_ID/slack \
  -H 'Content-Type: application/json' \
  -d '{"channel_id": "C0123ABCDEF", "since": "2025-01-01"}'

# Process it (extracts text, then runs fact extraction)
curl -X POST http://localhost:8080/api/sessions/$SESSION_ID/process

Screenshots

Testing

Unit tests for markup cleanup, timestamp parsing, and permalink building.

cd backend && go test ./internal/services/slack/ -v

---

Generated with Amp