Fix unused warnings in streaming components

[SteveGilvarry]

Motivation

• Reduce compiler warning noise from unused imports, variables, and struct fields to satisfy -D warnings CI policy.
• Make small, targeted changes to streaming and route code without altering behavior.
• Prepare codebase for cleaner CI runs and future refactors by removing dead fields and unused locals.

Description

• Trimmed unused imports and adjusted error mapping closures in src/handlers/hls.rs to use _err and removed unused HeaderMap/IntoResponse imports.
• Removed unused imports in src/streaming/hls/session.rs and simplified the use list to drop debug, SegmentInfo, and VideoCodec where not required.
• Simplified native WebRTC route wiring in src/routes/webrtc_native.rs by removing an unused native_routes local variable and keeping the authenticated/public route setup.
• Dropped unused struct fields: removed broadcast_capacity from ZmFifoReader in src/streaming/source/fifo.rs and removed config from WebRtcEngine in src/streaming/webrtc/engine.rs to silence dead field warnings.

Testing

• Ran cargo test --all-features; compilation proceeded but test execution stalled and the run was interrupted before completion.
• No unit/integration test failures were observed prior to the interruption, but the full test run did not complete.
• No cargo fmt or cargo clippy checks were executed in this change set.

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Codex Task