Route GORM's logger through slog and bound it (closes #178)

GORM's default logger printed the fully interpolated SQL to standard
output on every statement that returned an error, including a plain
record-not-found. On /webhook/{uuid} and on the login form the
interpolated parameter is client-chosen and unbounded, so an
unauthenticated client sized the operator's log, one line per request,
at no level the operator could turn down.

Every gorm.Open in the service now installs internal/gormlog, a
gormlogger.Interface over the service's *slog.Logger. Its lines take
the level the operator set and the handler internal/logger selected; a
record-not-found is not logged at all, since it is the expected
outcome on both of those paths and each handler already records its
own miss at DEBUG without the SQL; slow statements are kept at WARN
above the same 200ms threshold GORM used; and every value it emits is
spent through an encoded-byte budget.

That budget is internal/middleware's truncateLogField, moved to a new
internal/logfield package now that a second writer needs it. The move
is unchanged logic. MaxAccessLogLineBytes bounds a GORM line too, and
internal/gormlog asserts each line against the constant directly.

The third gorm.Open, in the archive writer, was not named in the issue
and had the same default.

README: the ceiling now covers GORM; the writers it does not cover are
named, including net/http's nil ErrorLog, fx's console logger and the
Go runtime, none of which carry a client-chosen value.

internal/delivery/target_database_archive.go

@@ -12,6 +12,7 @@ import (
 
 	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
+	"sneak.berlin/go/webhooker/internal/gormlog"
 )
 
 // archiveExpiryNever is the expiry sentinel (and default) that
@@ -282,7 +283,11 @@ func (w *archiveWriter) openMode(
 	}
 
 	gdb, err := gorm.Open(
-		sqlite.Dialector{Conn: sqlDB}, &gorm.Config{},
+		sqlite.Dialector{Conn: sqlDB}, &gorm.Config{
+			// Never leave this at GORM's default. See
+			// internal/gormlog.
+			Logger: gormlog.New(w.log),
+		},
 	)
 	if err != nil {
 		_ = sqlDB.Close()