refactor(dashboard): remove dead hooks/utils and update API docs

[AgilityB]

Description
Removes two dead library files from the dashboard that had no consumers anywhere in the codebase, and rewrites the stale API route documentation to accurately reflect the current state of the app.

Type of Change
🐛 Bug fix
✨ New feature / enhancement
📝 Documentation update
🔧 Chore / refactor
🧪 Tests only

Changes Made

• Deleted useApiList.ts
  — a client-side fetch hook that was never imported by any page, component, or utility. It was an earlier abstraction superseded by the inline fetch pattern used across pages today.

• Deleted fetch-list.ts
  — a lower-level fetch helper that useApiList was built on. Also never imported externally. Confirmed zero consumers via codebase-wide search before deletion.

• Rewrote README.md
  — the previous version documented only 5 of the 20 existing API routes and contained a "Adding Webhook Support" tutorial describing a route (/api/webhooks) that already exists and is fully implemented. The updated doc covers all routes in a structured reference table and accurately describes the real webhook endpoint behaviour, including its rate limiting, size guards, and response codes.

Test Evidence
No logic was changed — only dead files removed and docs corrected. Existing tests are unaffected.

pnpm typecheck # passes — no imports of deleted files existed
pnpm test # all existing tests pass

Checklist

• I have read CONTRIBUTING.md
• This PR is linked to an open issue
• pnpm typecheck passes with no errors
• pnpm lint passes (or issues are pre-existing and documented)
• pnpm test passes (all existing tests still pass)
• The dashboard starts and renders correctly (if UI or API code changed)
• The Discord bot still starts and responds to commands (if bot code changed)
• Docs site renders correctly (if docs changed): pnpm dev:docs
• No secrets, tokens, or API keys are included in this PR
• Environment variable changes are reflected in .env.example
• I have updated documentation if new behaviour was introduced

Additional Notes
The deleted files were identified by tracing imports across the entire codebase. No active code paths were modified. This is a noise-reduction change only.

Closes #324