fix: add missing requirements_realtime.txt referenced throughout install docs

[mvanhorn]

Summary

Adds the requirements_realtime.txt file that the install docs reference but that was missing from the tree, and pins the dependencies the realtime modules actually import: aiohttp plus aiohttp-cors (imported by the HTTP API and monitor dashboard), and websockets capped below the version that changed the server-handler signature the push service relies on.

Why this matters

docs/INSTALL.md points at requirements_realtime.txt in five places, but the file did not exist, so a fresh install following the docs could not set up the realtime stack. Beyond restoring the file, aiohttp alone does not pull in aiohttp_cors, and an unconstrained websockets resolves to a major version whose serve handler signature breaks the existing two-argument RealtimeDataPushService.handle_client, so both are constrained to what the code actually runs against. Reported in #26.

Testing

Added a test that asserts the requirements file exists and lists the imports the realtime modules need; it passes locally.

Closes #26