fix(store): pin connection pool so foreign_keys pragma sticks

SQLite PRAGMAs are connection-scoped, but database/sql uses a connection
pool. Without pinning to one connection, new pooled connections won't have
foreign_keys enabled, breaking ON DELETE CASCADE enforcement.

Also mark modernc.org/sqlite as a direct dependency in go.mod.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.mod

@@ -2,6 +2,8 @@ module git.dcglab.co.uk/steve/emcli
 
 go 1.25.0
 
+require modernc.org/sqlite v1.53.0
+
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
@@ -12,5 +14,4 @@ require (
 	modernc.org/libc v1.73.4 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect
-	modernc.org/sqlite v1.53.0 // indirect
 )