fix(backend): give each report file a name of its own (closes #61)

Report files were named by a millisecond timestamp and created with
O_EXCL, so two flushes in the same millisecond, such as a flush for
size and the final flush at shutdown, got the same name and the second
failed, losing its reports. Each name now carries a number after the
timestamp that counts the files written since the server started, so
names still sort by time and never repeat. The buffer reads the time
through a clock the new test stops, so its two flushes share one
timestamp on every run; the 1 ms pauses earlier tests used to dodge
the collision are gone.

Model: opus-5-5

backend/README.md

@@ -102,9 +102,12 @@ which `netwatch` owns.
 
 ### Report storage
 
-Reports are written as `reports-<timestamp>.jsonl.zst` files in `DATA_DIR`.
-Each file contains one JSON object per line, compressed with zstd. Files are
-created with `O_EXCL` to prevent overwrites.
+Reports are written as `reports-<timestamp>-<number>.jsonl.zst` files in
+`DATA_DIR`. The timestamp is in UTC to the millisecond, so the names sort by
+time; the number counts the files the server has written since it started, so
+two files written in the same millisecond still get different names. Each file
+contains one JSON object per line, compressed with zstd. Files are created with
+`O_EXCL` to prevent overwrites.
 
 ### Report limits