feat: query sensors only if they exist, batch sensor data requests

[aabhinavvvvvvv]

Summary

Fixes #642

The dashboard was rendering all 14 chart types unconditionally for every selected cell, firing up to 14 × N API requests on each cell selection — most returning empty data. This PR adds a sensor existence check and batch data fetching to eliminate wasted requests.

• Add GET /api/cell-sensors/ endpoint returning which sensor names exist per cell
• Extend GET /api/sensor/ to accept cellIds (comma-separated) for batch queries, returning data grouped by cell in a single response
• Dashboard now fetches sensor availability when cells change and skips rendering charts whose sensor type doesn't exist for any selected cell
• UnifiedChart now makes 1 batch request per measurement type instead of 1 request per cell × per measurement

[aleclevy]

@aabhinavvvvvvv This pr caused an api issue to arise with Marshmallow 'cellIDs'

marshmallow.exceptions.ValidationError: {'cellIds': ['Unknown field.']}

[aabhinavvvvvvv]

@aleclevy the root cause was that cellIds was missing from GetSensorDataSchema, causing Marshmallow to reject it. I've fixed this in the new PR #684 by adding cellIds = ma.String(required=False, load_default=None) to the schema and making cellId optional as well.