Clarify backend architecture boundaries

[therobbiedavis]

Summary

This PR cleans up the backend architecture boundary work from the #677 subtask. The main goal is to make the application layer describe app behavior and contracts, while infrastructure owns framework-specific and adapter-specific details like EF/SQLite, HTML parsing, image probing, audio tag writing, Data Protection, request context access, and SignalR/realtime wiring.

Changes

Added

• Application-owned ports for infrastructure concerns, including HTML extraction, Audible author-page parsing, cover image probing, audio tag writing, secret protection, request context access, realtime broadcasting, and realtime client inspection.
• Infrastructure implementations for those ports using the existing concrete libraries and framework features.
• Infrastructure endpoint/logging extensions that keep concrete realtime hub mapping and log hub initialization out of Program.cs.
• Backend architecture documentation describing the intended layer responsibilities and dependency direction.
• AI/contributor guidance reminding agents to review repo docs before making changes.

Changed

• Moved infrastructure-shaped behavior out of listenarr.application and behind application interfaces.
• Kept realtime transport details in infrastructure while Program.cs calls named infrastructure composition helpers.
• Centralized several commonly used backend usings and removed unnecessary per-file directives.
• Updated tests to use the new application ports instead of reaching through infrastructure details.
• Cleaned wording so comments/logs describe realtime behavior without leaking hub implementation names outside the infrastructure setup layer.

Fixed

• Removed package references from listenarr.application that belonged to infrastructure concerns.
• Fixed startup lifetime/DI issues uncovered while moving these boundaries.
• Preserved existing realtime event behavior while making the hub target explicit through the application abstraction.
• Stopped request context snapshots from trusting raw X-Forwarded-Host / X-Forwarded-Proto headers, so outbound notification URLs only use values normalized by ASP.NET Core forwarded-header middleware.

Testing

• dotnet build listenarr.slnx --no-restore
• dotnet test listenarr.slnx --no-build — 696 passed
• Push hook also ran backend format, frontend type check, and frontend tests — 378 passed
• Verified API no longer references concrete SignalR hub/log sink types directly
• Verified API controllers no longer reference Listenarr.Infrastructure.*

Notes

This is intentionally still a backend architecture cleanup, not a feature change. Program.cs remains the composition root, but the concrete realtime hub mapping and log sink initialization now live behind infrastructure-owned extension methods.

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[therobbiedavis]

Closing this in favor of the combined foundation PR: #686. The architecture boundary changes from this PR are included there.