fix(api): Increase ticker group limit

[itswcl]

Summary

• increase custom ticker group limit from 8 to 12
• move the numeric limit into shared-types so API and web use one source of truth
• update frontend max-group messaging to render from the shared limit

Verification

• npx vitest run src/tests/ticker-groups.test.ts
• npm run lint (apps/web)
• npm run build (packages/shared-types)
• npm run build (apps/web)
• npm run build (apps/api-server)
• git diff --check