Binary protocol

[gloskull]

Closes #406

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Motivation

• Reduce WebSocket payload size and bandwidth by replacing JSON envelopes with a compact protobuf-style binary envelope for high-frequency real-time events.
• Keep backward compatibility so unknown events and existing JSON payloads continue to work via a JSON fallback.

Description

• Added src/services/socket/binaryProtocol.ts which implements manual varint/fixed64/length-delimited encoding/decoding, event schemas for notification_created, course_updated, and message_received, and estimatePayloadReduction to compare JSON vs binary sizes.
• Updated src/services/socket/index.ts to encodeBinaryMessage on outbound emit and to decodeBinaryMessage for inbound ArrayBuffer/Uint8Array frames while preserving existing socket lifecycle and reconnection behavior.
• Added tests at src/__tests__/services/binaryProtocol.test.ts that validate known-event round-trip, unknown-event JSON fallback, and payload reduction metrics.
• Documented the protocol shape and usage in README.md under the new "WebSocket Binary Protocol" section.

Testing

• Added unit tests in src/__tests__/services/binaryProtocol.test.ts which exercise encodeBinaryMessage, decodeBinaryMessage, and estimatePayloadReduction.

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