fix: avoid division by zero when logging watch load rate

[snowyukitty]

Summary

• Measure watch-loading durations with time.perf_counter().
• Avoid dividing by zero when the clock reports a non-positive elapsed duration.
• Add a deterministic regression test for an empty datastore completing within one clock tick.

Root cause

load_all_watches() used time.time() and unconditionally formatted loaded / elapsed in both completion log paths. A fast load can receive identical clock readings, making elapsed == 0.0; an empty datastore then aborts startup with ZeroDivisionError while trying to log 0 / 0.0.

This was reproduced on Windows, where the wall clock has lower resolution, but the guard is platform-independent. The regression test replaces only this module's clock reference with constant readings, fails on master, and passes with this fix.

Testing

• Regression test before the fix: fails with ZeroDivisionError at file_saving_datastore.py:330.
• pytest -q changedetectionio/tests/unit/test_file_saving_datastore.py — 1 passed.
• pytest -q changedetectionio/tests/unit/ — 210 passed, 30 subtests passed.
• ruff check . --select E9,F63,F7,F82,INT.
• ruff check changedetectionio/tests/unit/test_file_saving_datastore.py.
• git diff --check.