fix: Reduce log verbosity for element state changes and WebSocket lag

[srperens]

Summary

• Change element state change logging from info! to debug! level (pipeline-level state changes remain at info for visibility)
• Change "Client is lagging" WebSocket warning to debug! level (expected behavior during pipeline startup bursts)
• Remove unused warn import from websocket.rs

Problem

During pipeline startup, hundreds of log lines were generated:

• Every GStreamer element state transition (Null->Ready->Paused->Playing) was logged at INFO level
• WebSocket clients received "lagging" warnings during startup bursts when they couldn't keep up with the flood of events

Solution

• Element state changes: moved to DEBUG level (pipeline-level changes remain at INFO)
• WebSocket lag warnings: moved to DEBUG level since it's not an error condition

Before/After

Before: Hundreds of state changed logs + dozens of lagging warnings per pipeline start
After: Only 3 pipeline state transitions logged per flow (Null->Ready->Paused->Playing)

Test plan

• Run RUST_LOG=info cargo run -p strom-backend and verify clean startup logs
• Verify pipeline state changes still logged at INFO level
• Verify no "Client is lagging" warnings at INFO level
• Pre-commit hooks pass (fmt + clippy)

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