fix: correct RAM overhead calculation and remove redundant assignment

[Devqo]

HordeWorkerProcessManager.__init__ contained two issues in the initialization of self.target_ram_overhead_bytes:

1. A redundant assignment that was immediately overwritten on the next line
2. A missing unit multiplier, the value was capped at 9 (bytes) instead of the intended target_ram_overhead_bytes (9 GB by default), introduced as a regression in 31d86e6a

This fix preserves the user-configured limit while ensuring it never exceeds 50% of total RAM.

Notes:

• target_ram_bytes_used (the property consuming this value) currently has no callers, so this is a non-breaking cleanup. The fix prevents incorrect near-zero headroom if memory enforcement is wired up in future.
• Pre-commit hooks passed. mypy skipped, project has pre-existing missing stubs for psutil unrelated to this change.