Add connector demos and fix WebSocket keepalive handling

[jillesvangurp]

Summary

• add a reusable local hub demo runtime plus GTFS and OpenSky connector demonstrators
• add uv-managed Python tooling and NDJSON WebSocket listeners for live validation
• fix hub WebSocket keepalive handling so active clients are not dropped due to stale read deadlines

Details

• move the local Dex/Postgres/Mosquitto/hub stack into connectors/local-hub/ with persistent bind-mounted state and dedicated setup docs
• keep the GTFS demonstrator wired to the shared local-hub runtime
• add an OpenSky demonstrator with airport fence bootstrap, live aircraft publishing, and listener/alignment scripts
• document the connector demos in the repository docs
• refresh WebSocket read deadlines on normal inbound messages
• add WEBSOCKET_READ_TIMEOUT and WEBSOCKET_PING_INTERVAL configuration for explicit liveness tuning
• add regression coverage and configuration docs for the keepalive behavior

Validation

• python3 -m py_compile connectors/opensky/*.py connectors/opensky/scripts/*.py connectors/gtfs/*.py connectors/gtfs/scripts/*.py
• uv run --project connectors/opensky python connectors/opensky/connector.py --env-file connectors/opensky/.env.local --once
• live OpenSky validation against the New York area with airport fences and NDJSON capture
• just bootstrap
• just generate
• just check

Closes #30