fix: negative TTL docstring, timedelta precision, and zero TTL warning

[emiliano-go]

Changes

Bug: Negative TTL docstring promises ValueError that never fires

CacheBackend.set() stated "Raises ValueError: If ttl is negative" but the
code silently treated negative values as "no expiry" (identical to ttl=0).

Fix: Updated the docstring: "None or a value of 0 or below means the key
will not be automatically expired."

Bug: Timedelta TTL sub-second precision loss

CacheBackend.set() converted timedelta TTLs via
int(ttl.total_seconds()), discarding sub-second values.
timedelta(milliseconds=500) rounded down to 0 (no expiry).

Fix: When a timedelta is provided, compute milliseconds via
int(ttl / timedelta(milliseconds=1)) and pass px= to redis.set().

Bug: Warning for default TTL=0 (unbounded growth)

Storing entries without an expiry (TTL=0, the default) silently persists them
forever with no indication to the caller.

Fix: Added logger.warning(...) in both _store_cache_entry() and
CacheBackend.set() when no TTL is set.