Stop TestEvictionRunsOnPeriodicSchedule racing the evictor (closes #183)

The test wrote each variant file and then inserted its accounting row by
hand while the evictor was running. A reconciliation pass between the two
steps adopted the file first, and the hand insert failed on the unique key.

The test now writes the files only, while it holds the test database's
only connection, so the evictor's startup pass waits after walking the
still empty variant directory. A periodic reconciliation pass then adopts
the files and the eviction pass after it evicts them; no write-pressure
notification fires. The test waits until two of the three files are gone,
then checks that usage is within the limit and that no row points at a
missing file.

Model: opus-5-5

TODO.md

@@ -29,6 +29,15 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 `TestEvictionRunsOnPeriodicSchedule` no longer races the evictor
+  (closes #183): it wrote each variant file and then inserted its accounting row
+  by hand, and a reconciliation pass between the two adopted the file first, so
+  the insert failed. It now writes the files only, while holding the test
+  database's only connection so the evictor's startup pass waits after walking
+  the empty variant directory; a periodic reconciliation pass then adopts the
+  files and the eviction pass after it evicts them. No other test in
+  `internal/imgcache` inserts a row by hand after starting the evictor. Test
+  only.
 - 2026-10-04 deployment guide and example Caddy config (closes #89):
   "Deployment" in `README.md` says what the reverse proxy in front of pixa must
   do (terminate TLS; pass `Host`, `Origin` and `Referer` on unchanged; set