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 all of the statement's arguments, 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