refactor: extract ws handlers to separate files to reduce handler.go size
- Extract job handlers (handleQueueJob, handleQueueJobWithSnapshot, handleCancelJob, handlePrune) to ws/jobs.go (209 lines)
- Extract validation handler (handleValidateRequest) to ws/validate.go (167 lines)
- Reduce ws/handler.go from 879 to 474 lines (under 500 line target)
- Keep core framework in handler.go: Handler struct, dispatch, packet sending, auth helpers
- All handlers remain as methods on Handler for backward compatibility
Result: handler.go 474 lines, jobs.go 209 lines, validate.go 167 lines
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