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

[aabhinavvvvvvv]

Fixes #642

Re-submission of #650, which was reverted due to a Marshmallow validation error.

Root cause of revert: cellIds was missing from GetSensorDataSchema, causing ValidationError: {'cellIds': ['Unknown field.']} on batch requests. Fixed by adding cellIds = ma.String(required=False, load_default=None) to the schema and making cellId optional.

Summary

• 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 fetches sensor availability when cells change and skips rendering charts whose sensor type doesn't exist for any selected cell
• UnifiedChart makes 1 batch request per measurement type instead of 1 request per cell × per measurement

[codecov]

Codecov Report

❌ Patch coverage is 21.68675% with 65 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
frontend/src/pages/dashboard/Dashboard.jsx	13.95%	37 Missing ⚠️
backend/api/resources/cell_sensors.py	26.31%	14 Missing ⚠️
backend/api/resources/sensor_data.py	0.00%	10 Missing ⚠️
frontend/src/services/sensor.js	33.33%	4 Missing ⚠️

❌ Your patch check has failed because the patch coverage (18.00%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

Flag	Coverage Δ
backend	70.14% <27.27%> (-0.52%)	⬇️
frontend	50.59% <18.00%> (-1.59%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
backend/api/__init__.py	80.91% <100.00%> (+0.29%)	⬆️
backend/api/schemas/get_sensor_data_schema.py	100.00% <100.00%> (ø)
...end/src/pages/dashboard/components/chartConfigs.js	100.00% <100.00%> (ø)
frontend/src/services/sensor.js	26.66% <33.33%> (ø)
backend/api/resources/sensor_data.py	22.22% <0.00%> (-4.20%)	⬇️
backend/api/resources/cell_sensors.py	26.31% <26.31%> (ø)
frontend/src/pages/dashboard/Dashboard.jsx	33.09% <13.95%> (-3.51%)	⬇️

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[aabhinavvvvvvv]

Hey @jmadden173, conflicts are resolved and the branch is rebased onto main. The Codecov report shows low patch coverage — happy to add tests if you'd like. Let me know what you think after review.