Adds backend/engine/sweep.py with three sweep strategies:
- GridSweep: exhaustive enumeration of all parameter combinations
- RandomSweep: N random samples from parameter ranges (list, min/max, step)
- GuidedSweep: top-K exploitation + random exploration from previous results
Features: bounded parallelism via asyncio.Semaphore, token budget enforcement,
Redis-based pause/resume/stop control flags, sweep-level event publishing.
36 tests in test_sweep.py covering config generation, helpers, and full sweep execution.
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