compress_data: warn when quiet_start == quiet_end (silently disables compression)

[pbitzer]

Problem

Setting quiet_start and quiet_end to the same value (e.g., both 0) results in compression never running, because _is_quiet_time() evaluates an empty range:

if self.quiet_start <= self.quiet_end:
    return self.quiet_start <= current_hour < self.quiet_end

0 <= current_hour < 0 is always False.

This is logically correct (zero-length window = no window), but it's a silent failure that's easy to hit when trying to configure "always on" compression.

Proposed fix

• Add a warning log at startup if quiet_start == quiet_end, since this effectively disables compression
• Document that quiet_start=0, quiet_end=24 is how to enable 24/7 compression

Context

Hit this in production on mj41 and mj43 (HAM-82). Compression was accidentally disabled for ~24 hours.