refactor: replace Combine WebSocket streams with AsyncStream

[Sandesh282]

Description

The previous WebSocket layer used Combine PassthroughSubject / CurrentValueSubject to propagate live events from LiveWebSocketService up through the repository and ViewModel layers. This created a mixed-concurrency model where Swift Concurrency (actor, async/await) owned all state, but Combine owned all event propagation — forcing callers to bridge between the two worlds with .sink + Task { } wrappers that are easy to get wrong.

This PR removes that split entirely. The entire event path is now 100% pure Swift Concurrency.

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Changes

WebSocketService.swift — Protocol

• events: AnyPublisher<WebSocketEvent, Never> → eventStream: AsyncStream<WebSocketEvent>
• connectionState: AnyPublisher<...> → connectionStateStream: AsyncStream<WebSocketConnectionState>
• WebSocketEvent and WebSocketConnectionState now conform to Sendable so they can safely cross actor boundaries inside continuations

LiveWebSocketService.swift — Concrete Implementation

• Removes PassthroughSubject / CurrentValueSubject
• Replaces with a [UUID: Continuation] dictionary pattern guarded by NSLock — each call to eventStream returns an independent AsyncStream backed by its own continuation, so multiple concurrent subscribers (e.g. ContestRepository + SubmissionRepository) don't interfere with each other
• connectionStateStream yields the current state immediately on subscription (cold observable equivalent of CurrentValueSubject)

Repositories — ProfileRepository, ContestRepository, SubmissionRepository

• Remove import Combine
• Replace bindWebSocketEvents() Combine .sink closures with withTaskGroup + for await event in wsService.eventStream loops
• Manual throttling (1s for ratings, 500ms for verdicts) replaces .throttle(for:scheduler:latest:)
• Each repository now vends its own AsyncStream properties (e.g. ratingUpdated, verdictReceived) backed by the same continuation dictionary pattern

ViewModels — ContestViewModel, ProfileViewModel

• Remove import Combine
• Replace Set<AnyCancellable> + .sink/.assign with [Task<Void, Never>] observation tasks
• All UI state mutations hop to MainActor.run { } explicitly, making the threading contract visible in code

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Before / After

	Before	After
Event transport	AnyPublisher (Combine)	AsyncStream (Swift Concurrency)
Combine imports	6 files	0 files
Subscriber pattern	.sink + store(in:)	for await loop in Task
Thread dispatch	.receive(on: DispatchQueue.main)	await MainActor.run { }
Multi-subscriber support	Shared subject (single multicast)	Independent continuation per subscriber

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Testing

Build-verified on Xcode (simulator). No functional behaviour changes — this is a pure infrastructure refactor. Existing SwiftUI previews and app flow unchanged.