feat: Complete health check TODO - verify /web folder exists

[Starosdev]

Summary

Complete the TODO in health_check.go to verify that the /web frontend folder is populated.

Why

The current health check only verifies InfluxDB and SQLite connectivity. If the frontend folder is missing or empty, the health check still passes but users see a broken UI.

File

webapp/backend/pkg/web/handler/health_check.go (lines 23-24)

//TODO:
// check if the /web folder is populated.

Acceptance Criteria

• Health check verifies /web folder exists
• Health check verifies /web folder contains expected files (e.g., index.html)
• Returns appropriate error if frontend is missing
• Unit tests added