refactor: extract duplicated Slack/PacificTime code into SharedKernel utilities

[devin-ai-integration]

Summary

Eliminates ~490 net lines of duplicated infrastructure between the YouTubeComments and YouTubeUploads modules by extracting three shared utilities into Hookline.SharedKernel:

1. SharedKernel.Common.PacificTime — single source of truth for PT timezone logic (quota resets at PT midnight). Modules keep thin internal facades delegating to the shared impl:

// Module facade (preserves internal API + namespace)
internal static class PacificTime
{
    public static string TodayKey() => SharedKernel.Common.PacificTime.TodayKey();
    public static TimeSpan UntilMidnight() => SharedKernel.Common.PacificTime.UntilMidnight();
}

2. SharedKernel.Slack.SlackSignatureVerifier — the identical 36-line HMAC-SHA256 + constant-time compare implementation was duplicated verbatim in both modules. Now a single public sealed class in SharedKernel; modules register it directly via DI.

3. SharedKernel.Slack.SlackSocketModeServiceBase — abstract BackgroundService encapsulating ~130 lines of WebSocket transport (connection open, receive loop, ACK, reconnect, error isolation). Modules subclass and implement only DispatchEnvelopeAsync:

// Comments: routes only "interactive" envelopes
protected override Task DispatchEnvelopeAsync(IServiceProvider sp, string? type, JsonElement payload, CancellationToken ct)
{
    if (type != "interactive") return Task.CompletedTask;
    return YouTubeCommentsProviderEndpoints.DispatchBlockActionsAsync(payload, ...);
}

// Uploads: routes "events_api" + "interactive"
protected override Task DispatchEnvelopeAsync(...) => type switch { "events_api" => ..., "interactive" => ... };

Net diff: +179 / −668 lines. All existing tests and architecture constraints pass unchanged.

Link to Devin session: https://app.devin.ai/sessions/672d4d38efcc4a0cac0d2a6c297166ca
Requested by: @skyp1nus

[devin-ai-integration]

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