Fix/replace axios with fetch slack

[SahilJat]

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Changes

Closes #7431

Replace @slack/web-api with native fetch calls to eliminate transitive Axios dependency which triggered multiple critical/high CVEs.
1)Rewrote api/slack-client.js to use fetch directly instead of WebClient.chat.postMessage
2)Rewrote e2e/slack-e2e-reporter.ts to replace postMessage and files.uploadV2 with direct Slack API calls (3-step upload flow: get upload URL → upload bytes → complete upload)
3)Removed axios override from package.json overrides block

No functional behaviour changes — all Slack messaging and file upload behaviour is preserved.

How did you test this code?

1)Confirmed package.json no longer contains the axios override
2)The existing E2E pipeline will exercise slack-e2e-reporter.ts naturally on the next test run with failures
3)api/slack-client.js can be smoke tested by triggering a Slack message in a dev/staging environment with SLACK_TOKEN set

[claude]

Claude Code Review

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[SahilJat]

The Django migration failure (relation "app_analytics_analyticsmodel" already exists) in the Python test is a pre-existing flaky CI issue unrelated to this PR. This PR only touches api/slack-client.js, e2e/slack-e2e-reporter.ts, and package.json (frontend/Node files). No Python/Django code was modified.