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 new test flushes pairs
until one falls within one millisecond; the 1 ms pauses earlier tests
used to dodge the collision are gone.

Model: opus-5-5

TODO.md

@@ -23,6 +23,11 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: report file names can no longer collide (issue #61): each is
+  `reports-<timestamp>-<number>.jsonl.zst`, where the number counts the files
+  written since the server started, so two flushes in the same millisecond, such
+  as a flush for size and the final flush at shutdown, each get a file of their
+  own instead of the second one failing
 - 2026-09-29: bounded the report endpoint (issue #20): `POST /api/v1/reports`
   still needs no credentials, but each client address, as resolved through
   `TRUSTED_PROXIES`, may send `REPORTS_PER_MINUTE` (default 60) reports a