Seed the retention tests 50 rows per insert, not 500 (closes #198)

internal/database was the slowest test package, and three retention tests were nearly all of it. Most of their time went to seeding thousands of rows 500 per INSERT: the SQLite driver finds each parameter's value by scanning the statement's arguments from the first until it reaches that parameter's, so binding grows with the square of the parameter count, and a 500-row insert carries several thousand. Seeding 50 rows per insert stores the same rows. No test case or assertion changes. The header of script/test records the new figures; its 90s timeout stays.

Model: opus-5-5

script/test

@@ -27,7 +27,11 @@
 # Those figures predate tests hashing the admin password at 1 MB instead of
 # 64 MB (https://git.eeqj.de/sneak/webhooker/pulls/404). After that change, in
 # a cache-defeated build at host load 44-109 (2026-10-02), internal/handlers
-# took 8.5s and the slowest package was internal/database at 15.8s.
+# took 8.5s and the slowest package was internal/database at 15.8s. Once its
+# retention tests seeded 50 rows per insert instead of 500
+# (https://git.eeqj.de/sneak/webhooker/issues/198), internal/database took
+# 7.3s and the slowest package was internal/handlers at 8.1s to 10.0s, at host
+# load 25-48 (2026-10-02).
 #
 # -p 4 -parallel 8 keep the run under 2 GB of memory: at most four test
 # binaries build or run at once, each with at most eight parallel tests. Under