Optimize Live dashboard

[ezio-melotti]

This PR optimizes the Live dashboard by introducing two major changes:

• The list of sensors is fetched once at the beginning, and then once every 5 minutes (instead of being fetched every 5s together with the latest reading like before);
• The SQL query used to retrieve the latest timestamp now assume that the timestamps are already sorted and retrieve the last value directly, instead of scanning the whole content of the tables finding the most recent timestamp.

[Copilot]

Pull request overview

This PR targets Live dashboard performance by reducing how often sensor metadata is fetched and by changing the “latest reading” SQL lookups to avoid scanning sensor tables.

Changes:

• Frontend: cache /api/sensors metadata and background-refresh it every 5 minutes instead of refetching on every live poll.
• Backend: change “latest timestamp / latest reading” queries to fetch the last row via descending id order.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/simoc_sam/api.py	Updates “latest” SQL queries to use ORDER BY id DESC LIMIT 1.
frontend/app.js	Adds sensor-metadata TTL caching and background refresh behavior for the Live dashboard.

Comments suppressed due to low confidence (1)

src/simoc_sam/api.py:174

• /api/latest orders by id instead of timestamp, which changes semantics from “most recent reading” to “most recently inserted row”. If any readings are inserted out of chronological order, the API will return a stale reading even though newer timestamps exist in the table.

            try:
                row = conn.execute(
                    f'SELECT sensor_id, timestamp, {", ".join(metrics)} '
                    f'FROM {sensor} ORDER BY id DESC LIMIT 1'
                ).fetchone()

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